Paul Kedrosky pointed to Lifehacker’s Better Gmail. I hope Paul won’t mind if I just reprint his entire Better Gmail is Gmail 2.0 post here, since it says it all, elegantly and forcefully:
Gina at Lifehacker has created the #1 most important tool any Gmail user needs. It’s an add-in, called Better Gmail, combining all the major Greasemonkey hacks for Firefox-based Gmail users, and it turns Gmail from indispensable into … really indispensable.
I’m already using most of the pieces as discrete Greasemonkey hacks, but bundling them all like this is a work of genius. Just install it. Now.
Lifehacker also has a good tutorial on using the extension.
A really interesting side note: as Better Gmail is a firefox extension, it’s not available for IE users. It’s an interesting twist on the browser wars. In the old days, Microsoft and Netscape fought to lock in users with incompatible extensions. Here we see the same thing happening simply because that one platform is open and the other is not. The users themselves are evolving the browser. There’s no intentional incompatibility (and it’s not in the browser itself). It’s just that one browser is getting more capable than the other as a result of its user community.