Ghosts N Goblins, i just like how the clouds look.
Zelda: Wind waker Tech and Texture Analysis *picture heavy* - Polycount Forum
not game art, but the different levels of the backgrounds is expressed through size and color, especially in the return of the jedi poster on the right.
rayman origins. hand-painted look. also blurry foreground is a pretty cool idea. Haven't played it yet though.
the game Chester, by Brilliant Blue-G. I highly recommend looking at more screenshots of this game, it's crazy how good the art is. it's like playing a cartoon. http://www.desura.com/games/chester
Proteus by Dave and Ed. I think that's 3d done with solid colors as textures/materials? Anyway it looks cool and i'm looking forward to actual video footage. http://www.visitproteus.com
Really Big Sky by http://bossbaddie.co.uk. one of the most beautiful use of particles (and bosses) i've seen in a while. watch a video for it plz.
borderlands 1 by gearbox.
Resonance by Omaha's own http://xiigames.com/. retro pixel feel of the sierra point'n'click games of the 80s and 90s.
Sumioni: Demon Arts. Very artistic ps Vita game by Acquire in japan. Painted and awesome looking.