Two LaTeX packages to add comments to documents while working together with others.

Package todonotes

The todonotes package allows inserting text boxes on the margin of the page. They are linked to the position in the text, where the \todo{} is entered.

An example can be seen at WriteLatex.

Package trackchanges

This package displays notes at the page margins similar to the above mentioned. However, it not just collects to do's, but supports a history of changes.

How this can be used is best seen in an example at WriteLatex.