PBWiki

PBWiki's promotion for doubling your wiki space is ending so here's my 2 cents:

I got a wiki early on for my high school friends to use for events, info, and other stuff that's collaborative. It's pretty convenient and I think most of 'em check it time to time or update something. And once y'get used to the wiki language, it's not too bad to update something. Quite useful.