Archie McPhee has the best items I've ever seen just for fun. Their supplier (Accoutrements) makes awesome famous people actions figures too, if you're into that sort of thing.
RunRunRun Dongoolines by Kiki Miki Hitomi is one of the most original shirts I've seen. When I first saw the front, it seemed like just a neat shirt. But the back makes a statement by showing how deforestation hurts not just us, but the little apple people too. It's one of those shirts that makes people think (and look) twice.