Drawar

Threadless
Snorg Tees

There's my 2cents.

Noisebot. I <3 it, as they say. T-Shirt Hell, too, of course. :D

I would have to agree with Threadless for this one.

Depending on your tastes:

UneeTee
Vintage Vantage
Palmer Cash
Busted Tees
Nerdy Shirts

I love a good t-shirt. These are some of my favorites.

I have to add Thinkgeek. Not exclusively shirts, but still some excellent ones.

Definitely Threadless, and they've had a sale all week, and is still going on. I think I've order 6 shirts this week already. :)

I just added an entry to my 'coolest cycling t-shirt' list. Yeah, I know I'm pimping the blog, but hey it's relevant, ok?!

Original list
Some significant updates to the list.
Most recent addition to the list.

Their cool, no matter if you're a cyclist or not....

You can try BeyondHelp.net they have some pretty funny t-shirts and some pretty offensive ones as well.

Check out zazzle.com, too. I've been wary of these types of places ever since I saw the poor quality of some of the Cafepress merchandise, but zazzle's stuff is awesome.

Ropeadope has some pretty sweet stuff as well.

I like RedBubble.

Designbyhumans and allmighty's if you're looking for threadless-like winning T-shirt. Bountee if you're in UK, or A better Tomorrow if you're in German

check out my blog if you have free time, I write a t-shirt blog :)

Heres one for all things cycling shirts related, got some pretty good info too.