I'm a big fan of the t-shirt. It lacks the pretentiousness of the button-up shirt. It's dead easy to put on. It provides an unbroken surface on which to print ace designs. It's versatile, being compatible with both other t-shirts and jumpers. But because I like a good t-shirt, I equally abhor a bad one – and there are many, many bad ones in the world.
Threadless is a very cool site that lets people like me put the world to rights. Can't find a t-shirt you like? Design one yourself and submit it to the site. Others will vote on the quality of your design, and if they deem it of a suitable quality, you'll win cash and prizes, and Threadless will sell it direct. Then you'll win the respect of your tee-loving contemporaries.
There's also a community aspect to the site. If you submit a picture of yourself wearing a Threadless tee, you get credits towards buying another. It's a nice way for Threadless to get some free models, and for you to get some free clothes.
Threadless is based in the States, but it ships worldwide. Good on it.