Chrome: Google's adding offline support to Gmail, Calendar, and Docs. The offline support comes via the Offline Google Mail extension, available now in the Chrome Web Store.
Offline Gmail
To get started with Offline Gmail, install the Offline Google Mail extension. Once installed, Chrome should open a new tab; click the newly installed Gmail Offline button, select Allow offline mail, and click Continue. Once everything's up and running, you'll be able to read, respond to, and search your email when you don't have an internet connection. When an internet connection is available again, the extension will sync your messages and other "queued actions" with Gmail.
The other thing you'll notice off the bat is that the offline version of Gmail uses the attractive, two-paned tablet version of Gmail (similar to the one you can enable in your inbox using Gmail Labs). Some of your muscle-memory will still work there—for example, the j and k shortcuts still navigate between messages—but beyond that it's mostly a point-and-click affair. (Hopefully they'll add more shortcuts in time.)
Offline Calendar
To enable Offline Calendar, visit Google Calendar, click the settings cog in the upper right corner, and click Offline. You'll be prompted to install the Google Calendar extension for Chrome. Currently, when you're offline in Gcal, you can view events and RSVP to appointments. Not bad, but that doesn't appear to include creating events, which seems like an important option. Look for that improvement in time.
Offline Docs
To enable Offline Docs, visit Docs and, likewise, enable Offline from inside the settings cog. You'll be asked if you want to enable offline mode, then you'll be prompted to install the Google Docs extension for Chrome. Like Gcal, the offline options are limited. You can only view docs and spreadsheets when you don't have a connection. But Google says they know editing is an important feature, and they're working on adding it.
This Is New?
If you previously used the now-defunct Google Gears to get offline access for Google Apps, this may seem like we took one step back, then another forward. The difference, as far as I can tell, is that the new offline support uses local storage features added in the HTML5 spec. So it's in many ways the same as a feature we had and then lost, but it's a little more future proof.
It's unclear whether or not other browsers (like Firefox) will get these features via extensions, but if I had to make a guess, I'd say probably not. At least not from an official Google extension.
