Adobe Buzzword
Adobe's vision for cloud word processing. Interesting because it includes integration with Acrobat and other post-production tools (useful for printing).

Google Docs
The most popular environment for online collaboration.

Hackpad
A hybrid between a wiki and a shared note-taking tool. Very light-weight. Probably not the best choice for documents that require extensive formatting

Microsoft Office Web Apps
Windows Live? Microsoft Skydrive? Office Web Apps? Microsoft's confused attempt at online collaboration. I am sure they will get it right eventually. Potential to integrate with MS Word.

Primarypad
Self-described as "Superfunky Collaborative Writing For Schools." Based on the open-source Etherpad. Notable for the ability to "rewind" through the drafts of your document. Basis for the now-defunct Google Wave and many other projects. A great choice!

Zoho
A full range of MS Office-like cloud applications. Second only to Google Docs in scope and popularity.