Pathfinder from Paizo Publishing is a new RPG compatible with Dungeons & Dragons 3.5. That means you can upgrade to Pathfinder to get a more modern system for your game, and you don’t have to throw away all your 3.5 or OGL books
The core rules are currently free, because they’re in playtest/beta, get them at Paizo.com as a PDF.
It uses the OGL, the Open Gaming License, so it should be easier for third party publishers to sell supplements for than fourth edition DND.