Google Docs supports comments.
First you select text, or place the cursor somewhere in your document.
Then you click on the “Insert” tab and finally on “Comment”. (“Ctrl-M” also works as a keyboard shortcut.)
You can now type directly into the comment area. The comment area is clearly delineated by a color of your choosing.
I’ve attached an example (oars_abstract) produced with Google Docs. It’s rendered here in PDF so that the comment can be viewed.
For additional help with comments, you can have a look at the Google Docs & Spreadsheets Help Center.
As I’ve blogged elsewhere, this is an example of annotation in the context of word processing.
In the case of Google Docs, this is precisely where I’d love to see an integration with Google Notebook. More specifically, extend the Google Docs notion of a comment by allowing for a Web-addressable comment. In delivering a comment that can be identified by a URL (or even better a URI), we’re closer to having an annotation.
And while we’re at it, one more thing. I’d like the resulting annotation to make use of XPointer 
