In the past I've played and GMed, in no particular order:

• D&D (Tiny bit of 2E, quite a lot of 3E and 3.75 and some 4E). It is the classic after all.
• Mutants and Masterminds. Does the job. The GM is the important part here.
• GURPs. Never again... Not after 7 wolves took 2 sessions to kill.
• Deadlands. Fantastic world.
• Savage Worlds. I don't think it lived up to it's promise. The unskilled roll mechanic kills it for me.
• Exalted (1E and 2E, Solar, Lunar, Dragon-blooded and Fair Folk). A massive world is balanced on the head of a pin and your characters are the force for change on a huge scale.
• Old World of Darkness (Vampire, Mage and Werewolf) The world is stranger than anyone knew. All that strangeness makes for fascinating explorations.
• New World of Darkness (Vampire, Werewolf and Changeling) nWoD does a very good job of bringing in more darkness. It encourages engagement with a small sandbox that can feed into the personal horror themes of each game.
• D20 Modern. This was the game that proved that d20 had been played out to me.
• Call of Cthulu d20. Palatable mechanics. It doesn't quite fit the theme thanks to using classes and levels instead of skills.
• WFRP 3E. Cards. Everywhere. Don't let that put you off though, the dice mechanic helps you narrate more detail about how your actions go ahead, beyond simple success and failure.
• Spirit of the Century. Pulp action and adventure. If you can go from not knowing anything about the game to the Pyramids of Mars in an hour you have this game and a great GM.
• 6d6. Tried it, didn't like it. FATE does a much better job of achieving the same goal.
• FATE. I haven't been able to think of anything this system can't handle for me.
• Legend of the Five Rings. Detailed and fun world that keeps evolving. The mechanics are bound into the themes incredibly well.
• Fiasco. The horror... oh the horror. Being chased by syphilitic man-eating penguins through the antarctic ice. And that was before the intermission.
• Scion. The Gods are still here and they are very randy. You're the result, and your grandparents have woken up to take the world back. It also doesn't assume anybody is going to stay hidden (which is very refreshing in urban fantasy).