I have been testing a gmail panel for a friend of mine, and things are going well enough that it looks like he is ready to release some early bits.  If you use desktop sidebar, and gmail, this is a natural fit.  It has replaced the gmail notifier for me, and is working pretty well.

Basiclly, it shows your inbox much like an outlook panel, including showing the first few lines when you hover over a message. 

Check it out here