Google Notebook lets you share a notebook with a group of people, in a similar way you do this in Google Spreadsheets. These people can add notes or edit notes written by other people, but you can't know who wrote something or when they added a note. You can add collaborators even if the notebook is public.
There's also a gadget for the personalized homepage that looks similar to the Firefox extension (or IE plugin).
The new feature may be useful for a team project, or if you want to explore the web with someone.
Other new features include an improved recycle bin for notes and an easier way to organize your notes, by moving them using drag and drop.
I think we'll see more collaboration features in Google tools, as they're not difficult to implement and are a great advantage for a web tool.