I like reading my emails with fixed-width fonts. Gmail by default does not provide this feature. So, I have to tweak my browser, Firefox, to accomodate this. Early on I edited Firefox *.css to do this. But that would mean I have to edit those CSS on every computer I am on. Besides, other people who use the same computer (and firefox) may not like the configuration.
I am glad that I found this greasemonkey script (from persitent.info) that can be used to toggle the fonts in gmail. After installing this script, there is a "Toggle Font" link in the right side of my email. I can click on it to switch to fixed fonts and back as needed. The nicest thing about this is that other people can use the same Firefox with their own configuration.
Now, if only greasemonkey works on other browsers (Safari, Camino, Flock, Opera). Yes, I have five browsers in my Apple iBook and Mac mini!