Firefox/Chrome/Opera/Safari/IE: Gmail's already got several great themes, but if you've never been satisfied with Gmail's clutter, the Helvetimail user script transforms Gmail into a Helvetica-based, stripped-down interface.
(Click the image above for a closer look.)
Helvetimail is a logical—if obvious—step in the latest craze of applying minimal designs using the Helvetica font to popular web applications (see previously mentioned Helvetical, Helvetireader and Helvetwitter). To use the script, you'll need to enable the Minimalist theme in Gmail's themes settings. Then, depending on what browser you're using, you'll have to install the user script. (Firefox users, if you've installed Greasemonkey, just click the user script link on the Helvetimail page to install. For the rest of you, the script's author has instructions for various browsers.)
Helvetimail is probably the least attractive of the Helveti-bunch of user scripts we've seen (I love the look of Helvetical), but if you're a big fan of the user-contributed, minimal Helvetica designs, it's worth a look.
Helvetimail [Josef Richter]