There's a new class of apps emerging, what I call Social Art apps. These are apps that let you create something, then see and rate creations by others.
Besides offering the tools to create, all of these apps allow you to see and rate content by others from within the app itself and see leader boards of content and creators. Many also allow you to comment and post links on social networks like Facebook and Twitter.
Here are some of the more interesting ones:
Doodleymedium: simple painting
audience: kid-centric
Spray Canmedium: graffiti
audience: youth
Sketch Clubmedium: drawings
audience: serious painters
Mixelmedium: clip art
audience: casual
tappr.tvmedium: visuals for music
audience: music fans