FEAR
I've been playing the F.E.A.R pc game for a couple days now - trying to work my way through the single player story. It's really good... and really difficult. I finally decided to use the Quicksave feature last night, which is making things easier. The firefights are intense in this game... and the effects are stunning. They've managed to capture the chaos of the environment as it's getting torn up during the gunfight.
Bullet time is very cool. I'm anxious to try this out in multiplayer. I would say the single player experience is about on par with HL2 technically... although it doesn't carry the same loftiness to the story that Half Life does.