Just to reiterate, I think Stikipad is the best hosted wiki implementation I’ve used. It helps that I can email the developers, ask for something, and have it 2 days later. They’ve recently changed the features in the free version, so give it a whirl.
Second is 9cays. Yes, they’ve unfortunately chosen another [0-9]+[::alpha::]+.com domain name, but I’m getting used to it. The idea is that you can have a normal email conversation with a group, and have copies of all messes held in “Conversations” at 9cays. Like little impromptu mailing lists. The best part is that it’s *very* non-disruptive. Simply CC your message to a 9cays address and the rest takes care of itself. Good stuff.