Have you ever wished the game you're playing could do something else? Why can't it tell you where an item is, or give you coordinates on a map? Maybe a warning about what a boss is about to do, or an easier way to cast the same spell over and over? You are not alone; other players have wished the same thing. Some of them have even written a small program to grant that ability. These are called AddOns.
Curse.com is the leading source for AddOns. They have AddOns for various games, including Warhammer Online, Runes of Magic, and Age of Conan. However, more than 75% of the AddOns they house are for World of Warcraft. AddOns are created by players, and published for other players to use. They are not created or supported by the related game's publisher, but unless they violate rules of the game, they are allowable. They are not a hack, and they do not interfere with the game's code. They simply make use of functionality already in the game.
A popular AddOn sometimes becomes a part of the game. One AddOn for WoW, QuestHelper (below), helps locate quest objectives on the world map. This function is scheduled to be added in game in patch 3.2.
There are literally thousands of AddOns for World of Warcraft. If there is something you want, the functionality is probably there, but finding it can be difficult. The top used AddOns are listed, and there are even lists broken down by use (PvP, Dungeons, Professions) and by class. Another source for determining which AddOns to download is your guild. Some guilds require members to have certain AddOns. It is important to align your AddOns with your guild's, as there are multiple AddOns for a similar function, but they will differ in how they are used.
Downloading the AddOns is not as difficult as it might seem at first. It is highly recommend that you download the Curse Client. This a program saved to your hard drive. From this client, you can find and download other AddOns. You can also update the AddOns you have already downloaded, simply by clicking on it. Updating AddOns will need to be done after every patch.
Below are a few suggested AddOns that are helpful, or entertaining:
QuestHelper - Don't spend an hour looking for that one item to click on, that your quest log says, "Is North-West of here by the furbolg camp." QuestHelper will mark your map with the exact spot to go to. Need to kill some mobs? Collect twelve items that drop off a different mob? Turn in a quest in a new area? All of these locations will be marked on your map. There will also be a dotted line showing a faster order to do these objectives. Don't want to keep looking at your map? No problem, QuestHelper also puts an arrow on your screen, pointing you towards your closest objective, and tells you how far away it is. Nice to have when starting out, but a must have when you've hit level 80 and are leveling up alts.
Deadly Boss Mods - Most guilds will require you to have this prior to running raids with them. A nice to have in Heroic dungeons as well. It gives warnings about what is about to happen in the boss fight, when the boss is about to cast something in particular, or more adds are joining the fight. Complete with a timer, so you can plan accordingly. If you are the only one who has it in a group, even if you've never done the fight before, you can warn the other party members what is about to happen, and you'll look like an expert.
Omen Threat Meter - Another must have for raiding, and even for heroics. When in a fight with a mob or boss, Omen puts a table on the screen showing how much Threat each person has. The creature will attack the person with the most threat, also known as aggro. Ideally, the tank has the most threat. If you are DPS, and your threat is going up faster than the tank's threat, you will know to slow it down, or switch targets, so you don't pull aggro from the tank. Not as useful for healers, as they are targeting the people in the party. It might be useful for a healer who has a tendency to steal aggro, to check the tank's targets, and make sure they are not on the threat list.
HealBot – The premium tool for healers. A screen shows all of the raid or party members, along with a health bar showing how much life they have. The healer can mouse over this bar, and heal with just a click of the mouse. Different heals can be assigned to different clicks (left-mouse, middle-mouse, or right-mouse button). There is no more clicking on the portrait at the left, then going down to the Action Bar to click the appropriate spell.
Gatherer - As the name implies, it is useful for those in the gathering professions, specifically Herbalism and Mining. Anytime you find and use a node, it marks it on your map. You can look at your map, see where nodes were, and go there to see if there is currently anything to mine or pick. This could have limited usefulness, and be completely useless if you had a photographic memory, but it has another function: information sharing. You can share your Gatherer with someone else, and every node marked on every map of yours goes to them, and vice versa. The more people do this, the more information that is exchanged. There must be people out there who have every known node mapped.
Peggle (or Bejeweled) - WoW has some downtime. You might be waiting for a raid to start, or on a twelve minute flight path to Winterspring, and are just sitting, staring at the screen, with nothing to do. Peggle has the cure for that boredom. A puzzle game can be brought up (and set to automatically come up when on a flight path). Now you can while away the time playing Peggle. There is also Peggle Loot, instead of rolling on an item in a dungeon or raid, people can play Peggle against each other, the one who scores higher gets the loot. Bejeweled is also available, a better known, but similar, puzzle game.
Peruse Curse.com and find the AddOns that are right for you.