Just gotta share this website with you (if you don't already know it).
Threadless is an awesome website with wicked cool shirts (also now extending into other garments), all featuring user-designed images/slogans etc.
I have 8 or 9 threadless shirts, and I LOVE them. They're so different from the general muck I tend to see in most shops, and I frequently get comments/questions about them when I wear them out.
Here's a wee selection of some of the latest designs that I've got my eye on.
Where Dreams Come True
Deliquesce
Sky Thief
Go get buyin', folks!