
Recent changes to the 4th Edition Game System License (GSL) of Dungeons & Dragons now allow publishers to re-release third edition products with certain revisions.Specifically, "Section 6" was removed which prohibited publishers of third edition Dungeons & Dragons products to produce similar products for 4th edition.
According to Wizards of the Coast's GSL Frequently Asked Questions:
The decision was made after listening to the concerns of the industry and community. Many publishers were concerned about adopting the GSL because of Section 6. We want publishers to create products under the GSL, not go around it, so we removed Section 6 to make the GSL easier to adopt.
Geek Related provides a blow-by-blow account of the differences between the two versions of the GSL, ultimately concluding "Producing third party supplements for Dungeons & Dragons Fourth Edition is now viable."
Goodman Games is leading the pack with updates of its 3.5 Dungeons & Dragons products. They've begun the slow process of revising prior products to make them compatible. The first wave of revised 3.5 PDFs are available now, including roughly the first dozen Dungeon Crawl Classics as well as the Judges Guild adventures.
Goodman Games plans to release the rest slowly over time as they revise the original files. You can purchase them from RPGNow.com, Paizo.com, YourGamesNow, or E23.