I have been playing around with designing T-shirts on Spreadshirt.com, and I just added a new one to my fancy store. (Is it just me, or is this the greatest expression ever?)
My T-shirt designing skills are a little weak, I think, but it's really fun, so I'm going to get better at Illustrator and keep cranking them out as I have time. So far I just have shirts I think my kids would like. Let me know if you have any requests or ideas!