Internally, we have tried many different wikis in an effort to collaborate and get us literally on the same page. We have settled on PBWiki. It’s not the most feature-rich (twiki has a tonne of plugins) or popular (mediawiki has to win this just from wikipedia alone) wiki, but it’s extremely easy to use, and seems to have a sensibility that is very amenable to the way we work. They’re offering to double our file space, so here’s a token link-love to them. Thank you, PBWiki.
p.s. I do think their grassroots marketing campaign is brilliant and one we should learn from.