Gmail now works offline without a plugin, but only in Google Chrome.
The new feature, part of the new Gmail, is really quick to set up. Head to Gmail’s Settings, then the Offline tab, as shown above. Check “Enable Offline Mail” and you’re done, though you can configure a few things how many day’s worth of emails should be stored on your computer and what should happen to your data when you log off of your Gmail account.
You can now visit Gmail in Chrome without internet access, which makes is great if you’re away from Wi-Fi for a while. It’s particularly useful for Chromebooks, which can’t really run local email clients (at least not until Linux apps run on Chromebooks.)