Today Gmail Labs released a new feature that bridges the gap between desktop and web-based applications like never before: Offline Gmail. You can now access your Gmail from your browser any time, whether or not you're online.
Offline Gmail uses Google Gears to download all of your email to your desktop (every last gigabyte). Using Gmail in offline mode is exactly the same as using it normally—except that when offline mode is enabled in your browser, Gmail is that much faster.
Composing emails in offline mode puts them in your outbox, and those emails are automatically sent once you're back online. Offline Gmail even has a very cool (though poorly named) "flaky connection mode," which is sort of like a middle ground between online and offline Gmail. While in flaky connection mode:
[Gmail]uses the local cache as if you were disconnected, but still synchronizes your mail with the server in the background. Our goal is to provide nearly the same browser-based Gmail experience whether you're using the data cached on your computer or talking directly to the server.
The upshot to flaky connection mode is that your computer doesn't have to hit Google's servers to access old emails, which generally will mean a faster, more responsive inbox.
I've been using Offline Gmail for a few months now, and I noticed the speed differences immediately. It takes a while for Gears to download all of your old email, naturally, but once it has, it keeps your offline cache in sync with your online email pretty seamlessly. Check the offline Gmail FAQ for more, and if you give it a try, let's hear how it's working for you in the comments.