The webshop Thinkgeek.com offers several cool geek clothes.

One of my favourites is the Wi-Fi tshirt which is powered by three batteries and shows if theres any Wi-Fi signals nearby.
http://www.thinkgeek.com/tshirts/generic/991e/

Another cool product is the “There is no place like 127.0.0.1″ tshirt which can be found here:
http://www.thinkgeek.com/tshirts/generic/5d6a/

That’s some great geek clothes!