A friend in Aussi introduced me to Threadless a while back and I think it's awesome how anyone who joins the community can submit designs. There really are some brilliant illustrators out there! Check it out http://www.threadless.com
They even have wall applications for your favourite designs!
In response to my earlier post about the Keep Calm and Carry On posters...
I love the fact that a lot of them also address social stereotypes or issues, in an expressive and lighthearted way.