Writely
This is slightly out of date, but registrations for the collaborative word-processing tool, Writely, are now open again.
Writely is basically a combination of an online word processor which also allows documents to be shared and edited by multiple people at the same time. It borrows collaborative concepts from wikis and applies them nicely in a what-you-see-is-what-you-get environment. I find it particularly useful because I can collaborate with people who would find editing wikis difficult because they have to learn wiki syntax, which is particularly difficult due to the unpopularity of plain text e-mails.
Give it a try if you find a need to use it.
The world says, So do I.