Zmarties

Greasemonkey is a Firefox extension that allows you to easily write site specific enhancements.

This has been used to add a persistent searches box for Gmail, and also a Toggle Font link to switch to displaying the current message in an fixed pitch font.

Mihai Parparita, the author of this elegant enhancement to Gmail, happens to work for Google, but has produced this independently of that fact - though you would think that he may be in a reasonable position to get this noticed and rolled into a future Gmail version.