Huddle

After many clunky false starts from the good and the great, online collaboration actually works now - try Huddle (huddle.net). The feature set is already good, and I know from talking to the London-based team who are making waves with this tool that the best is yet to come, including a developer API. iCal and Facebook integration are just the latest additions...

(The basic one-huddle package is free - and if you are a Charity, you can apply for a free corporate account with fifty huddles...)