Join 500,000+ artists on ConceptArt.Org.
Its' free and it takes less than 10 seconds!
Seems to be mostly hate speak and scribbling, but if you jump between the servers you can usually find some people drawing in a relatively sane manner.
Its dirt simple, one line width and color picker in the lower left corner. 180 seconds to draw then it wipes the board clean.
Net kiddies probably make this one nsfw unfortunately.