Dungeon World is an indie game based off the Apocalypse World game. The current edition is the Red Book, covers levels 1-5 and has everything you need to get you started. The PDF costs $5. The full version will be finished and released some time in 2012.
The game attempts to bring back some of the nostalgia you remember playing D&D 20 years ago, or create it for the first time if you missed out on that era. You kill monsters, take their stuff and explore dangerous locations. The rules are fairly simple and quick to pick up, though it plays a bit different from a lot of RPG's. The style of play it encourages is much more free form than a modern D&D 3.X game, or Pathfinder.
In a normal game, when the PC's encounter the plot, things might get a little screwy. In Dungeon World, when the PC's enter the mix, things get really screwy. In a recent game that I played in we encountered a magic rock. The wizard in the group used the ability Spout Lore to ask the question "How do I prevent it from blowing up?" The DM hadn't prepared for it blowing up, but it was now fact, it could blow up. He gave an answer... which allowed us to later blow it up, it was awesome.
If you're looking for a simple, easy to play game that works hard to capture the basics of what you love about old school D&D, this is it. It's an unfinished product, but the minds behind it and collaboration going are doing great work. If you play the game, post a report of your experience (on a blog, forum, wherever), then e-mail the creators of the game, they'll give you access to the Adventurer's Guild, and you'll be part of the people who are giving input into how the game develops.
It's funded (within 20 minutes of being posted), but the rewards and stretch goals include several adventures only available to backers. I highly recommend this game, one of the best pickup and play experiences I've had in RPGs since I started playing nearly 20 years ago.