I like Wikispaces and have used this successfully with students who learn to use it easily. Teachers can use the user creator so age is not a problem, student email addresses are not required.
Links on wikis including Wikispaces here.
For mind maps a very simple tool to use is Bubbl.us - see diagrams page here.
I like Wikispaces too, but my district blocks them. I've tried to have them unblock it but to no avail. I have found success with Mindmeister though for mindmapping and PBWorks for collaboration.
Thanks for the response.
Have you tried Google docs for collaboration? I really like it, and it's something your students will probably need to use in the future. (Google for Educators gives teachers added options in terms of administration, etc.)
This thread might have some viable mind-mapping options for you.
Stretch Your Digital Dollar
A lot of people seem to like Webspiration (because it is nearly identical to a paid program called Inspiration) for mind mapping, namely because it can convert your mind map into an outline. If you want to incorporate outlining, I'd recommend Webspiration. Might be a little complex for younger students, though. I would try Wallwisher instead, if outlining isn't a factor. Or maybe an online whiteboard, like Scriblink?
Thanks for the heads up on Webspiration. I like it. Now I can give my students a choice between a mind map or outline. I'm going to introduce this on Monday. Thanks again.
Talking of mind maps reminds me of a tool I rather like, have you seen SpicyNodes?
I wrote about it here.