I like the OGL Conan RPG, which I think does a decent job of depicting the world of Robert E. Howard's pulp fantasy novels using D20-style rules. Some of us remember the AD&D1 version of the game, as well as the well-written GURPS sourcebook. I also recommend Dark Horse's current Conan and Red Sonja comic books.
I agree that Green Ronin's Thieves' World is excellent for fans of that fantasy anthology series, although comparing the back-alley intrigues of the city of Sanctuary with the epic barbarians of Hyboria is a bit like comparing apples and oranges. Like the recent game based on the Black Company mercenary-themed books, they are both "low fantasy," where magic is mysterious and rare and swordsmanship a necessary survival skill.
Less successful, IMHO, was the D&D3.0/D20 adaptation of Melnibone, based on Michael Moorcock's Elric stories. It had good dark fantasy flavor, but the rules and descriptions didn't pull me in, and that version wasn't well supported. Now, if only any of these were made into a decent television/movie series, we'd be all set! As much as I love the "Lord of the Rings," "Narnia," and "Harry Potter" films, there are more subgenres of fantasy yet to become popular...