Here are my greasemonkey scripts.
Add access keys for navigating around mailing lists.
Add "Collapse" and "Expand" buttons to thread views which allow you to collapse/expand the tree of posts.
Convert the plain-text quotes in emails into HTML blockquotes and style them accordingly.
This script also detects and styles signatures to mails, and is slowly evolving to undo some of the damage of HTML mail too.
use DLs for header lists
Convert the unordered-list of headers into a DL (descriptive) list. Apply some style to make it take up less space. Finally, hide most of the extraneous headers.
If you use mutt, you might like Martin Krafft's mutt-mailto script