I got a request on one of the blog comments for a stealthy game other than Thief: Deadly Shadows or the Splinter Cell series. But are there really games other than those?! Yes! Not to fear!
Last year, I posted about Hitman: Contracts. If you're looking for stealthy, then Hitman is definitely a must have. You can play a game in pure stealth mode. I'm not quite that refined... (ahem) but it might be perfect for you.
There are stealth elements in a lot of games. One of my favorite games was Beyond Good and Evil. Beautiful game. Fun. Lots to do. Story. It has stealthy elements, but I wouldn't really call it a stealth game. Games like Psi Ops: Mindgate Conspiracy also have some stealth.
But for hardcore stealth: Thief, Splinter Cell, Hitman.
Go get 'em...