Let's face it, Bible publishing is big business.
While most Christians in the history of the world go their whole lives without their own copy of Scripture, we Americans can walk into any bookstore and have access to more copies of the Bible than was available in every town in the first century combined!
Yet this is not always a good thing.
There are some amazing study Bibles out there that are invaluable in helping readers of Scripture go deeper in their faith and knowledge than they ever dreamed possible...and there's also a lot of fluff at best, and garbage at worst, sitting on the shelves waiting to claim the cash of eager Bible buyers who may not know better.
So how do we distinguish between them all?
The purpose of a study Bible is to provide background information, insights and other data about the text of Scripture that help the reader understand the original meaning and, to a lesser degree, how to apply that understanding to the modern Christian life. I say "to a lesser degree" because "application" is not always something that can be provided to a mass audience. Rather, it's the Holy Spirit's job to stir the heart of the believer to apply the truths He's written in the pages of Scripture in their daily life (at least, this seems to be what Jesus thought He would come to do...check out John 14-16 for a more detailed discussion!).
That being said, I'm compiled a list of study Bibles that I readily recommend to my students and church members, as well as a list of "study" Bibles that are better left on the shelf. This is all totally subjective, some might say, but that's the beauty of being the one on this side of the keyboard! :)
[Note: Study Bibles come in all different translations, and the list below does not speak to the quality of the translation being used, just to the study material surrounding it. Also, the binding I recommend is almost always whichever one is cheaper. A paperback Study Bible can be turned into a very durable long-lasting study Bible with just a couple of sheets of contact paper. But if you want to pay $70+ for genuine premium leather binding and cut indexed pages...it's your money. But consider whether or not it might be better to buy two (or even three) of the same Bible in cheaper bindings and give the extra(s) away to someone who can't afford their own. WWJD, ya' know...]
RECOMMENDED STUDY BIBLES:
Life Application Study Bible (NIV or NASB)
One of the overall best study Bibles for the money. Combines background and theological insight with applicational insight for one’s daily life. 
Zondervan's TNIV/NIV Study Bibles (TNIV or NIV)
A great balance of background notes, illustrations, and cross-references make this an excellent study Bible for the money. Not as in-depth on life application notes, but good background info, timelines and charts make up for it.
Quest Study Bible (NIV) A perfect study Bible for those new to Biblical studies or those with questions regarding certain hard-to-understand passages or concepts. Thousands of questions sent in by readers are answered in the blue margin on each page by various Biblical scholars and teachers.
ESV Study Bible (ESV) This is one of the most comprehensive study Bibles on the market at this time. One of the best features is the Appendices section in the back which contains over 200 pages of articles and essays on various aspects of Biblical studies. Though slightly Reformed in theological orientation, the notes reflect scholarship and theological views from across the spectrum of Biblical Christianity. It also includes access to the online version for free.
Archaeology Study Bible (NIV) Produced in partnership by Zondervan and Gordon-Conwell Theological Seminary, this Study Bible is the best source of background information on the world of Scripture currently in print. Full color articles contain insight that is unavailable anywhere outside of commentaries and scholarly resources. [HIGHLY RECOMMENDED!!]
NET Bible w/ CD-ROM (NET) A completely new translation by scholars done entirely online, the NET Bible contains over 60,000 translation, background, and application notes submitted by scholars and teachers from all over the world. A downloadable version is available for FREE at www.bible.org/netbible. The print version contains incredible satellite maps of the Holy Land.
The Apologetics Study Bible (HCSB) Though it has fewer notes than any of the others, Holman’s contains many excellent articles and notes on intellectually interpreting and defending the faith as well as introductory biographical sketches of famous Apologists in Church history. It is a good study Bible for those who like the HCSB translation or are interested in Apologetics.
NLT Study Bible (NLT) The study notes in this one provide excellent insight from a wide range of evangelical scholarship and theological positions. It is recommended over the NLT Life Application Study Bible for those who prefer the NLT’s more readable translation.
RECOMMENDED PARALLEL BIBLES - Parallel Bibles are excellent for readers who don't know Greek and Hebrew because they let you see how different translations handle the text. Thus you are able to see the possible range of meaning and intepretation inherent in the text.
Today’s Parallel Bible (NIV,NASB,KJV,NLT) This Bible offers the best range of translations from across the spectrum. NASB and KJV seek to be word-for-word translations, whereas the NLT seeks to convey the overall meaning of the text in understandable language. The NIV lies somewhere in the middle of these.
TNIV/The Message Remix Parallel Bible - A more convenient size makes this a good parallel Bible that can be carried anywhere. The TNIV translation is an excellent update of the NIV and the Message Remix, while not a reliable Bible translation, often gives a fresh perspective on the text, particularly the narrative sections of Scripture. (Available in various bindings)
RECOMMENDED INTERLINEAR BIBLES - Interlinear Bibles are the closest an English-only reader can come to a "literal" translation. Each Hebrew, Greek or Aramaic word is printed with its most literal English equivalent just beneath it.
The Interlinear Hebrew-English OT, Ed. By John Kohlenberger, III - A great one-volume interlinear containing the Hebrew text (BHS 4th ed.) with word-for-word translation beneath each line. Also contains the NIV in a parallel column.
The New Greek/English Interlinear NT - An excellent interlinear containing the Greek text (USB 4th ed.) with word-for-word translation beneath each line. Also contains the NRSV in a parallel column. (Available in various sizes)
RECOMMENDED DEVOTIONAL BIBLES - Devotional Bibles are usually nothing but fluff and pretty pages. However, if you MUST have a devotional Bible, the following two are actually pretty good, despite a lack of study material or background information.
Strive (TNIV) A great Men’s devotional Bible without all the clutter of normal devotional Bibles. The text is presented in paragraph format to make for easier reading with excellent articles and discussion questions throughout. (Available in various bindings)
True Identity (TNIV) Like the Strive Bible, the True Identity Bible contains good devotional articles and discussion questions aimed at women, while also avoiding all the clutter and fluff of other women’s devotional Bibles. (Available in various bindings)
NON-RECOMMENDED STUDY BIBLES
Study Bibles by Individual Christian Teachers, Authors, or Theological Viewpoints - A good study Bible should provide more insight than a single individual author can contribute. It should also provide notes that are as objective as possible and that are not aimed at reading something into the text, but rather at getting the true meaning out of the text. Study Bibles that have the names of famous Christian authors, teachers, or speakers are often lacking in solid background notes, or reflect the theological bias of the individual editor or theological point of view. Thus, one’s money is better spent on a study Bible other than the ones below:
Charles Stanley’s Life Principles Bible (NKJV)
Joyce Meyer’s Everyday Life Bible (Amplified)
T.D. Jakes Woman Thou Art Loosed Bible (NKJV)
The Maxwell Leadership Bible (NKJV)
The Blackaby Study Bible (NKJV)
The Ryrie Study Bible (various translations)
The Scofield Study Bible (various translations)
The MacArthur Study Bible (various translations)
The Dake Annotated Reference Bible (KJV) [Strongly discouraged]
The Spirit Of the Reformation Study Bible (NIV)
The Reformation Study Bible (ESV)
The Wesley Study Bible (NRSV)
The Evidence Bible (NKJV)
Study Bibles by Liberal/Non-Orthodox Scholars - While these study Bibles contain many helpful literary and historical notes, they often operate from the view that Scripture is not Inspired in the orthodox sense of the word and contains errors, historical inaccuracies, false authorship, and/or other marks of a purely human product. They adopt scholarly assumptions from 19th-20th century European frameworks such as the Documentary Hypothesis for the Old Testament and the Q Hypothesis for the Gospels—both of which are unnecessary and antiquated by-products of post-Enlightenment skepticism. Theological and applicational notes are also kept to a minimum.
The Oxford/New Oxford Annotated Bible (RSV or NRSV) - However, out of the more liberal study Bibles on the market, this one is the most orthodox and faithful to the text.
The Harper Collins Study Bible (NRSV)
The New Interpreter’s Study Bible (NRSV) - Particularly unhelpful are the essays at the end such as "The Authority of Scripture", as well as the NIB's acceptance of the antiquated and illegitimage Documentary approach to authorship of the Pentateuch (see the "two flood narrative" section in Gen.6-9 for an example).
Gimmick Study Bibles - Some publishers, seeking a niche market, publish Bibles with certain “gimmicks”. Study Bibles should contain insight and background information in addition to any devotional or inspirational information. Many, unfortunately, simply contain little study material. For example, some attempt to color-code verses in Scripture based on subject matter. Unfortunately, many passages of Scripture cannot be simplified in such a way and speak to numerous subjects or situations. One’s money is better spent on a good study Bible such as the ones on the first two pages.
The KJV Rainbow Study Bible (KJV)
One Year Devotional Bibles (various translations)
The Green Bible (NRSV)
The American Patriot's Bible (NKJV)
There are many many more study Bibles on the market, of course. But these provide a good sampling of what to look for and what to avoid. So next time you walk into a Christian bookstore or Bible section, hopefully you'll be able to navigate your way to an excellent resource and avoid the stubble...
"If anyone builds on this foundation using gold, silver, costly stones, wood, hay or straw, their work will be shown for what it is, because the Day will bring it to light. It will be revealed with fire, and the fire will test the quality of each person's work. If what has been built survives, the builder will receive a reward. If it is burned up, the builder will suffer loss but yet will be saved--even though only as one escaping through the flames."
(1Corinthians 3:12 TNIV)