Mongrel Clothing has some nice t-shirts such as "walkies":

which reminds me of sniffer's row by threadless.com
Vintage T-Shirts, Tee Shirts, Tees at PalmerCash.com more in the (repetitive) vein of

I've just received an email that this company does not ship out t-shirts.
"You promote Palmer Cash (palmercash.com) t-shirts on your website. I
ordered t-shirts from that company, my credit card was charged, and I never
received the shirts. I contacted them by e-mail but never received a
response. No phone number is listed on their website. I did some research
and it turns out I am not the first person to be scammed. Check out
ripoffreports.com. Please do not promote this company so that others avoid
being rippoed off like me."
Funny tees from Bogus including the stunning "Romford Island":

gama-go - some very nice original graphic design / anime style shirts (US based). I like "Yeti Nepal".
molgam unique tees. Not the easiest site to navigate, but they are firefox friendly :)
CafePress.com sells "funny" shirts, like the profound Hugh G. Rection shirt.
Anti-Capitalist t-shirts from Disturbia Clothing like American Excess - only £19.99 (that's not very anti-capitalist).
Cillit Bang T-shirts - apparently this is some cult TV ad (probably popular amongst students).
BorderDogs on ebay has some funny shirts like "I know you are but what am I" (takes me back to school) and "kiss me I'm (almost anything)"
ConcreteHermit has a new limited edition design from Jon Burgerman, PEOPOLIS:
VintageShirts.com - more vintagey (surprise) Urban Outfitters style shirts such as...

One Off T .com - design your own t-shirts, and even get commission on them (rather liike the almighty threadless.com)