Web-based email is great because you can check it from any computer, but there's one little catch: it's inherently limited by your internet connection. Google is starting to roll out an experimental feature in Gmail Labs: offline Gmail.
So even if you're offline, you can open your web browser, go to gmail.com, and get to your mail just like you're used to.
Once you turn on this feature, Gmail uses Gears to download a local cache of your mail. As long as you're connected to the network, that cache is synchronized with Gmail's servers. When you lose your connection, Gmail automatically switches to offline mode, and uses the data stored on your computer's hard drive instead of the information sent across the network. You can read messages, star and label them, and do all of the things you're used to doing while reading your webmail online. Any messages you send while offline will be placed in your outbox and automatically sent the next time Gmail detects a connection. And if you're on an unreliable or slow connection (like when you're "borrowing" your neighbor's wireless), you can choose to use "flaky connection mode," which is somewhere in between: it uses the local cache as if you were disconnected, but still synchronizes your mail with the server in the background.
Offline Gmail is still an early experimental feature and Google has made it available in Gmail Labs for those of you who want to test it out.
The offline Gmail will be available to everyone who uses Gmail in US or UK English over the next couple of days.