Document collaboration is becoming more important in the modern workplace. Instead of sharing documents via email or, even worse, via thumb drive, we can now very easily share and synchronously collaborate on documents stored in the Cloud.
Below, please find some of my favorites:
Google Docs (must have a Google or GMail Account to create a new document)
Acrobat.com (must create an account. Uses Flash. Includes videoconferencing tool: Adobe Connect for up to three participants
TitanPad (lets people work on one document simultaneously. No sign up required)
CrocoDoc (allows you to upload, view, markup and collaborate on Word, PowerPoint and PDF documents online. No sign up required but larger documents can be uploaded with sign up. Also allows markup of Web Sites)
SkyDrive (for those in the windoze world, Microsoft has a service called SkyDrive that offers 25gig of storage and document sharing...not bad)