One of many militias in the war in the Democratic Republic of the Congo (formerly Zaire). In 2000, they were a maverick army fighting for the expulsion of Rwandan and Ugandan troops from the country. They're believed to number anything between 2000 and 20000. The current organisation succeeds a faction of the same name which fought against Mobutu Sese Seko as part of a rebel alliance including local and Rwandan Tutsis whom they never liked in the first place.
Mai-Mai forces are based in the north-eastern Kivu province and are known to have a large number of children as young as eight in their ranks. They employ traditional spears as well as more modern machine guns and believe that magic water makes them invulnerable to bullets. The name "Mai Mai" means "water" and some of them have been reported to wear shower caps and faucets in battle. If it weren't bloody reality, it could be a tale from Mad Max.
In January 1997, their erratic behaviour became such a liability that president Laurent Kabila ordered their disarming despite their being on his side but three years later they were again receiving government support. In March 2001, they were reported to be in training for peacekeeping operations under the terms of a ceasefire deal.
It's hard to separate the fact from the rumour here, especially regarding a maverick militia in a war where sides are often switched, the factions themselves are subject to change and the information that reaches the outside world is often contradictory.
Sources:
Christian Science Monitor
Business Day (South Africa)
Rädda Barnen (Save the Children Sweden)
Congokin Forum