I recently learned about def.i.ni.cion, a nonprofit clothing company with a mission and a message, started by Brian Yi. Basically, they've made it a point to offer conscious, socially responsible t-shirt designs that try to make a statement about the wearer and the world.
The "about" page on their site is pretty wordy, but basically, 20% of the price of your purchase goes to partner organizations involved in education, relief, clothing, food, loans, housing, job training, medical attention, and justice. Good causes, and cool designs. Learn more, and check out their designs here.