On a breif psuedo break from WoW, due to an influx of impressive single player games, and a lack of time to devote to Multplayer for a short bit. Following are some games I've tried lately...
Fable: The Lost Chapters (PC Version): Man, wow. I had brushed this game off, not being an Xbox owner at all, but once the PC version came out, I figured I'd give it a try. This game rocks, hard. Think the best parts of Zelda, Morrowind, and all your other RPGs of that nature, and jam it into one game. There are certainly a few things that could have made this better- if this had no load times (however short they already are) it'd really lend to the feel. I'm in love with this game like I havnt been with a single player release since KOTOR 1.
Warhammer: Dawn of War: THIS is how Warcraft 3 should have been. What a fantastic RTS- fun (customizable!) units, varied races, cool setting, and just plain carnal. If you love RTSs (which honestly, I don't, and I love it) pick this up!
I've tried a few others (Psychonauts, what I played of it, was disappointing. It seems like there was a lot of optimizing for PC they had left to do, as for a game that doesnt look great, it ran pretty choppily), but these two stick out. Notably Fable- a true classic, in my mind.