"TrackChanges is a package for collaboratively editing LaTeX documents. It allows multiple editors to make changes and add annotations to a document. These changes and notes will be color coded by editor and added to the compiled document. The TrackChanges GUI allows the author to quickly find and accept, reject or modify the suggested edits."

A simple but useful service to save and share bits of text online. No sign-up or logging in involved, though you can choose to password protect your snippet.

Online collaborative scribbling. A virtual whiteboard, if you like. Interesting, and potentially useful.