There are many great clothing sites out there, but a couple really stand out for me; Funkrush and Threadless. The prototypical customer for each site would probably be a designer themselves, and would appreciate some of the wonderful illustrative works featured on these t-shirts.
Threadless - A ‘Web2′ approach
Threadless has really built up an entire community around t-shirt designs. Any designer who thinks they’re worthy can make an account and start submitting their works, and the designs that make the cut are chosen by the community. Designs must receive a certain rating from the community before they’re printed. This process seems to work well, and it’s obvious that a lot of thought has gone into making Threadless as community-driven as possible.
Funkrush Quality
Funkrush takes a different approach to design. Their workforce mainly consists of well-known and respected illustrators from online communities. This ensures that only great quality designs are churned out, and seems to work great!
I’d advise everyone to at least check out these two great sites, who knows, maybe you’ll fall in love with their apparel as much as I have 
Images courtesy Funkrush.com and Threadless.com
Ahhh Threadless is my bestfriend when it comes to awesome tees. I’ve amassed quite a number of great shirts. I’m hoping to submit my one in the future, as well but for now, I’ll leave it to others.