Kudos to all who mentioned Little Big Adventure, although it's technically not an adventure game, more of a action/platform title. It's the first PC game I ever played and still one of my favorites!
Discworld Noir, already cited many times is wonderful too!
Here are a few of my favorites that haven't been mentioned yet (I think):
The older, lesser known Cryo games, Dragon Lore 2
Never finished this one though, it was hard, a mix of point & click QTE-esque actions sequences...
and Lost Eden
Beautiful and poetic, great characters (the narrator is a pteranodon!), great voice acting, great music. Check out the screenshots on the mobygames page!
Lastly, Versailles 1685, but I guess you have to be french (oui monsieur), AND love baroque music (tout à fait!), to find an interest in this one... http://www.mobygames.com/game/versailles-1685
Apart from that, there's Alternativa, by czech developer Centauri Production, which was recently released on steam. Non-linear storyline set in Prague a dystopian 2050... I've recently started playing it, it looks interesting. http://www.centauriproduction.com/alternativa.php
I apologize for the lack of screenshots, but I must confess I'm a noob: I don't know how to implement screenshots in my post...