Google Docs has a new feature, Compare Documents. This feature is access from the Tools menu.

When select, Compare Documents will ask for another document. This other document will be compared with the current document, and the differences will be written to a third document as suggested edits.

Compare Documents is very handy with long documents when collaborators have made a copy and worked from the copy instead of the original document. In the programming world, a programmer would create a branch of a document and work form that branch. When the document was ready, the changes would be merged back into the original. Compare Documents can do that for you, allowing you to create a copy (or branch in programmer speak) and then merge those changes back in to the original copy.