Recommend Me a Good Stealth Game

Darn, no mentioning stuff like Tenchu or Shinobido then. :(

Mark of the Ninja and Stealth Bastard Deluxe are both nice ;)

Metal Gear Solid is a really good one

I loved Alpha Protocol, but you don't have to play it as a stealth game (which I didn't because I don't like stealth games). Very underrated game that's a bit buggy. It seems like the bugs may cause more issues when you're trying to be stealthy, though (i.e. being seen through cover, etc).

I'd say you should also check out the original Deus Ex, it gives you the option of approaching everything stealthily or going in with rocket launchers. (I normally opted for stealth :P )

private infiltrator

Splinter Cell Chaos Theory is a very good stealth game. Thief 2 is my favorite of the 3 in the series as it improves a lot since the first. You could also take a look at Velvet Assassin which was fairly good even though it's a bit clunky and painfully linear. You can get it really cheap in a sale.

Dishonored is so good, you can complete the game fully non-lethal even (taking care of bosses and targets in more interesting ways :P).

Deux Ex and DX:HR are both great games for playing stealth as well, as is Mark of the Ninja and the Thief series.

Happy sneaking!

Dark is quite a bit of stealth, bad game though, very poorly developed, bit buggy and extremely overpriced.
Hitman (older series, not much the last one)
+1 for MGS
Amnesia? Outlast which is on pre-order would seem to adjust your reqs. There's more, a few on snes or psx, can't remember names tho, but you wanted PC/steam only so..

Invisible war is not good.
Everything I was going to suggest is already here.

Gunpoint!

Heard some good things about Doom 3's The Dark Mod. They're working on a standalone. So if you're like me and don't have Doom 3 you'll have to wait a little :(

EDIT: Read recommend stealth game and jumped the gun, didn't know it had to be available on Steam. SOZ.

tomb raider?

Any Splinter Cell game. Most of the early ones are without UPlay, if that's the only thing holding you back.

RL Payday. Self-explanatory.

My recommendation
Deus Ex - Stealth can be a bit broken, but it's fun to play due to how many option the games give you on how to take on missions.
Mark of the Ninja - I really don know what to say rather than it's a solid stealth game
Gunpoint (Puzzle platformer + stealth) - Not difficult in the stealth department since it's more of an puzzle platformer but it's a fun game none the less
Splinter Cell until Chaos Theory(?) - Very solid Action-Stealth games then the series went all actioney

I'd avoid
Bethesda RPGs - It's not real stealth no matter how you play it
Alpha Protocol - Don't get me wrong the narrative is amazing, but the stealth gameplay itself is broken meaning the AI will detect you no matter what.

I only knew the thief-series that you mentioned already :)

The original Deus Ex is amazing. Invisible War was a piss-poor sequel though.

The only games I can think of are as follows:
Fallout 3/New Vegas (While not stealth games, you can choose to play a very stealthy character as I did)
Monaco (I haven't played this one, but it seems to have lots of stealth)
Stealth Bastard Deluxe (I haven't played this one yet, but it seems to have lots of stealth, hence the title)
The Elder Scrolls V - Skyrim (As with the Fallout games, it's very much a choice, but oh so much fun)

It's been said a dozen times already, but Mark of the Ninja - it's a great stealth game.

half-life 3

metro last light, to get yhe epic ending you must finish the ggame sstealthy