Gallina: The GMail Blog
Here is a blogging system that uses your Gmail account as its backend. It uses “Starred” messages as the posts, and replies to those messages are the comments.
Written in PHP and uses libgmailer to connect to the Gmail account, it also makes use of XML/XSLT and has support for JPG and PNG attachments.
It is released under a GPL license. Check out the demo blog: http://jaws.com.mx/gallina-0.3/gallina/index.xml. It shows the number of invitations that are avaialble and how much space is being used.
Possible uses: a collaborative blog that works on e-mail? I may have to give this a test run.