I'm on a Basecamp group right now. I think it's excellent, but all I'm doing with it is uploading some files and checking images others post, but I seems like a pretty complete project management package. From what I've seen of it, I'd recommend it.

I also use Google groups, which has everything I would normally need to collaborate with others on line.

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I've used it extensively for international clients, remote designers, and as a course convenor.

I've found it to be great for pulling together groups because it is very easy to use and orientation/learning happens through actual use, which helps speed up the start up stage.

Every group I've shared it with ends up using it in their own way. I usually make some suggestions at the start and then it beds in as the project rolls on.

I used basecamp for a couple of years, but now I've switched to a self-hosted option.

--> The one I use is Collabtive, which is open source: http://collabtive.o-dyn.de/?lang=en

--> Previously I tried ProjectPier, which is also open source and quite good, but doesn't seem to be under active development: http://projectpier.org/

-->And a third one, commercial, but probably the most complete of the three so far, is activeCollab: http://www.activecollab.com/