Four

The Google Docs would be great for a writing assignment.  Students would work in a group to edit each others writing.  They can see how their document was edited and they can make changes and use their classmates comments to revise their writing before they publish.  They have the opportunity to work and help each other as they go through the writing process together.  I would also be able to add my comments and make suggestions. 
The Google Reader would be a great way to keep up with peoples updates and blogging.