I just bought a few shirts from Design By Hümans, which for some reason I had never seen before today. It's in the same vein as Threadless and such, although I found myself liking more of the designs at DBH.
The interesting thing is that after you buy shirts, you're instantly given a discount code for the next purchase. Even cooler is that it can be used by anyone else as well, which means I can tell you that for the next 10 days you can save 15% on shirts by using the code I was given: QXMPFL.
If you end up buying a shirt I also get $2 off my next purchase, which isn't too shabby. Check them out.