I’ve been looking fairly recently at Pathfinder, a game system extremely close to Dungeons and Dragons, but with (in my opinion) a significant number of improvements.
It is 100% compatible with D&D 3.0 and 3.5, but just has some nice fixes here and there. (Stealth being a single attribute in 4.0 was done long ago in Pathfinder. Class progression is usually smoother. Balance is shifted around a little).
Most impressive however to me, is how they have changed Sorcerer as Rob was saying. There is now actually a reason not to take a prestige class when you’re a sorcerer. Before, not multi-classing as a sorcerer was ALWAYS a disadvantage, without exception.
I think if I ever run any D&D games from now on, it’ll be Pathfinder as the base system.