This is a nice list of plug-ins for Greasemonkey that I’m using to make GMail more usable and productive. I hope one day GMail will integrate these.
Here’s the list:

Greasemonkey install
Next, you will have to restart Firefox so the Greasemonkey plug-in takes effect.

Conversation Preview
When this plug-in is enabled, right-clicking on any thread will display a bubble [...]