There are quite a few sites which offer applications for sharing and collaborating on documents, these are the sites that I have tried on different occasions, if you know of more please add:
GoogleDocs - online collaborative word processing
YourDraft - Online draft - shared editing - No registration neccessary
WriteWith - online collaborative word processing
Zoho Writer - online collaborative word processing
WriteBoard - online collaborative word processing
The applications share similarities and differences. So far I have found google docs very useful for working with my team - it enabled us to build and refine our curriculum online, from home, instead of spending extra hours in school.
I hope next year to use google docs with my students for both individual and group work.