Hmmm... well, the new(ish) Deus Ex has some good stealth in it. The Thief games are pretty good too, but they focus more on just sneaking, not killing.

Thief: Deadly Shadows

there's also assassin's creed but that may be too action based.

Hitman series, and the Thief series are a must for you then.

Velvet Assassin

The STALKER games, Thief, and my favorites, the Deus Ex games.

Not by any means waiting and knifing somebody, but Amnesia is possibly the best stealth game you can get.

Thief looks fun but I'm looking for a more modern setting ie. stealth suits and k-bar's, not cloaks and dagers.

Deus Ex: HR
You can choose how to play it, and one of the ways to play it is as a stealth game. It works really well that way (except for those fucking boss battles)

Does it matter if it's first- or third-person?

VTM: Bloodlines has some good stealth mechanics, if you have the right patches, although it's a rpg, an awesome rpg.

Alpha Protocol, best stealth game of the year :)) . Or you can try Dead to Spies, you are a spy but the stealth and the gameplay is fun and creative in my opinion.

Metro 2033.

Syphon Filter

F.E.A.R. has pretty nice opportunities for stealth imo, if you're into the horror genre.

Homefront.

Well, it's not out yet, but you should really keep an eye on Ghost Recon Future Soldier.

I watched the new Coop Walkthrough yesterday and it looks REALLY good! Coop Walkthrough

Do be honest, as far as the stealth genre (which really isnt that big) goes, Splinter Cell is as good as get. The games are great. There coming out with a new one and hopefully its more like Choas Theory or the xbox version (not 360) of Double Agent. Its called Tom Clancy's Splinter Cell: Retribution

Death to Spies

Well if you want some more amazing stealth, then I'd recommend The batman Arkham asylum and arkham city games. Apart from being some of the best stealth games ever. They are also some of my favourite games ever, along with Splinter Cell

A lot of these are good calls but the one game where I really thought "wow, this reminds me of Splinter Cell" while I was playing it is Cold War.

Batman, Hitman, weapons of fate, Project IGI :D

Rainbow Six Vegas 2, and opt for the stealthy option in every level.

Hitman, S.T.A.L.K.E.R. and of course

Metal Gear Solid (1 2 and 3, the 4th is more action shit).

Thief 1, Thief 2 and Thief 3. That's all you need ;)