You may remember early in the Arabian revolutions in Libya, an American student took the summer off college to fight in the revolution.

Adding to this, there is an entire online community of these people at the Black Flag Cafe. Outsiders label them as "war tourists," but the majority of them are journalists, war photographers, businessmen, and humanitarians/activists. The website was founded by Robert Young Pelton (whose wikipedia page is worth reading). He wrote a great book, that is filled with practical information.