According to Wikipedia:
Wikispaces is a hosting service (sometimes called a wiki farm) based in San Francisco, California. Launched in March 2005, Wikispaces is owned by Tangient LLC and is among the largest wiki hosts, competing with PBworks, Wetpaint, and Google Sites (formerly JotSpot).
A list of projects that have used Wikispaces:
Additional information and materials about Wikispaces: