You need a copy of Doom3 (for now, it might change in the future) and you get The Dark Mod.
It is a game heavily inspired by Thief, it is well done and it is plenty of good missions. The only problem is the lack of a overarching story like the one of Thief. http://www.thedarkmod.com
thank you all for the replies, i couldn't reach this post before because they changed the forum name and whatnot.
I'll try the dark mod, wich as of today doesnt need doom 3 anymore to play!!!
Also saddens me that there is no other game like thief... this reminds me to Tie Figther (the original, not the Vs game), no other game even similar to it was released ever...
According to that site though, most missions still need Doom 3 assets so you'll still need the game to play most missions.
And yeah, it's definitely a shame that nothing's really equaled Thief. At least we have a full trilogy instead of a single game like some excellent unique games.
EDIT: Although it looks like they're working on eventually going fully standalone, so that's good.