20 Essential Gmail Tips You Probably Don't Use (but Should!)
Back up and Use Gmail Offline
The reason it always seems like your Internet connection goes out at the worst possible time is because when you stop and think about it, there's really no good time to be without Internet access. Whether we're trying to play an online multiplayer game or collaborate on a cloud project, losing an internet connection sucks. That doesn't mean you can't still be productive, however, because Google has made it possible to use your Gmail account offline. Here's what you need to do.

Log into your Gmail account, click on the Settings link in the top-right corner (right next to your email account), and then click on the Offline link. You'll see a bunch of options for configuring Offline access, but the one you have to check for this to work is the 'Enable Offline Mail for this computer' radio button.

Configure the other options however you like and then press Save Changes. You should now see a pop-up window with a short description outlining what Offline access is. Read it and then press the Install button. This will open up yet another window directing you to install Gears. You need this app in order to access Gmail offline, so go ahead and click through the installer. Be sure to save your work because you'll need to restart your browser once Gears is finished installing.

After installing, you should be greeted with a pair of pop-up windows. One of them will ask permission for Offline Mail to use Gears (mark the checkbox and click Allow), and the other will ask if you want any shortcuts installed (select as desired).

At this point, the synchronization process will kick in. Depending on how much email you've acquired through the years, this could take a long while, even on a fast connection. Once it's finished, however, you'll be able to access all your mail offline just by clicking on any of the shortcuts you created in the previous step. And don't worry about remembering to sync in the future; Gmail will do this all on its own whenever you log in.
Gmail Backup
If you'd rather not use an email client like Outlook or Thunderbird, and don't want to install Google Gears for offline access, you can still maintain a backup of your Gmail account. Here's how.

Download and install Gmail Backup from here. Run the program, input your username and password, and choose a directory to save your mail. When you're finished, click the Backup button and Gmail Backup will do the rest. Should you ever need to view these messages offline, you can do so in Outlook and other clients that support the .eml file format. But what's even cooler is that you can restore your local emails back into Gmail just by clicking on the Restore button.
Integrate Gmail into Google Apps
Google does a lot more than just email, and that can mean wielding different accounts, tabs, or browser windows. If you're heavy into Google Apps, that can quickly become a drag, so here are a few ways you can integrate Gmail with other Google services.

We'll start off by mentioning iGoogle, which serves as your hub for most things Google, or your gHub (we made that up). With iGoogle set as your home page, you'll have quck access to a variety of services, including Gmail, Google Docs, Google Calendar, and a bunch of optional Gadgets, which you can access here.
Google Calendar

to integrate Google Calendar with your Gmail account, log into Gmail, click on Settings, and select the Labs tab. Scroll down and enable the Google Calendar gadget, and then click Save.
Google Docs Previews

Google Labs also has a gadget for previewing Google Docs. You'll find this entry just below the Calendar gadget. Once enabled, you'll be able to see previews of documents, spreadsheets, and presentations right in your email whenever someone sends you a link to a Google Doc.
Google Voice Player
There's no need to navigate back and forth from Gmail to Google Voice in order to play back your voicemails. Enable 'Google Voice player in mail' under the Labs stab and you'll be able to play back your voice messages from your email notification.