those that dont know:
Che Guevera, leader of several Latin American socialist movements, most notably Cuba's revolution. He supervised, and probably participated in, the execution of "enemies of the people." Eventually pissed off Castro (think Trotsky and Stalin) and ended up in some South American shithole, where he finally got the execution he deserved.
Fairly neutral wikipedia article.
Most of the morons who wear the shirts don't know shit about him.