The self-refreshing t-shirt from Domyos:

"The selfresh t-shirt is active, and cools your body during sport. It is covered with millions of hi-tech micro-capsules that contain cooling ingredients."

These cooling ingredients are generated by the friction caused when you move around in sport.

It's probably the only t-shirt I know that needs to the "recharged" after 10 washes (I don't know what that means). I'll give it a try one of these days.