Lots of great recommendations in this thread. The PC community is not the largest on VGC, but the few of us seem to have good taste. 
Although I don't know if TruckOsaurus likes p&c adventures, I'll repeat two which have already been mentioned in this thread and add a few more:
The Secret of Monkey Island: Special Edition (available exclusively on Steam atm)
Machinarium
The Book of Unwritten Tales
Ceville
A Vampyre Story
Tales of Monkey Island (if you're interested, get it directly from the developer's website telltale.com - it's cheaper there than on Steam)
The Whispered World (English version will be published early next year according to the developer)
Edna & Harvey The Breakout (English version available in Q4/09)
Also: Make sure to check GOG for lots of classics from all genres.
