Threadless is an online T-shirt store in which designs are solicited form the community and put up for popular vote. A few of the highest rated designs are then selected by the company for sale and the designers are paid a set fee.
The designs are usually gently humorous, wry or clever in some way.
Coty Gonzales, who writes a blog in which he reviews T-shirts, has posted a series of Threadless T-shirt designs with corresponding preliminary sketches, 76 Threadless T-Shirts From Sketch to Print.
I always enjoy seeing the preliminary sketches for finished illustrations or paintings.
(Images above, in sketch and final pairs: Justin White, Terry Fan and Eric Fan, Alvaro Arteaga Sabaini)