Threadless, I think, is one of most amazing social community right now. It is t-shirt design competition/store with over 300,000 users signed up and new tee shirt designs every week.
Every week designers and people who have interesting ideas to be realized as tee shirt print designs submit them for other users to vote. There are over 600 submissions every week but only four to six ideas are taken to be printed and sold. Winners receive 2000$ in cash and prizes and I think it is good motivation for designers. But the most amazing thing about Threadless is the community which gives a mass of positive emotions and fun.
How I wrote before Threadless have also the shop of tee shirts with the winning design prints on them. Now they have two categories of prints – graphic images and type tees and you need to check them out – there are a lot of cool things out there!