Gnutella

Gnutella is an open source P2P community. NullSoft initially created the client, however it was done without the approval of AOL. The program was made available in April 2000 for a brief amount of time before AOL executives caught wind of its existence. However, the cat was already out of the bag, as enough people had already downloaded the program. It was reverse engineered, and since then has enjoyed various degrees of success.