I recently played a charming, artful platformer called One and One Story after reading about it on Joystiq. It is quite brief --- 15 minutes, maybe --- but it has some unexpected mechanics, excellent music, and a strong, touching lovestory that make it more substantial than some AAA retail games. Also, the ending is wonderful. It made me smile and say, "Awww, that's so cute and clever". You can find it on Kongregate or Armor Games.