I had some time this morning to play with Google's Notebook (works with Firefox) and was impressed with its ease of use. Notebook sits quietly at the bottom of the screen and with a click you can open it up, type in a note or take a clip from any web page you are on. With the clip you automatically get the URL reference in your quote. Notebook is a collaborative tool which you can make private or public and invite others to add notes or edit yours. Changing the background colour of notes might be a handy way to identify notes related to subtopics.