20 Essential Gmail Tips You Probably Don't Use (but Should!)
5 Essential Gmail Downloads
Gmail Notifier

It's tough to imagine using Gmail without Gmail Notifier. This little utility sits unobtrusively in the systray and lets you know when there's a new message in your inbox. It also serves as a gateway to your inbox - just right-click and select View Inbox.
Free, toolbar.google.com/toolbar/gmail-helper/
Fuser

The hardest part about being a social butterfly in cyberspace is managing all those different accounts. It's easy to waste an entire morning just catching up on current events through Gmail, MySpace, LinkedIn, Friendster, and Twitter. That's where Fuser comes in, which is a catch-all front-end that combines all those accounts (including multiple accounts through the same service) into a single, manageable inbox.
Free, www.fuser.com
Gmail S/MIME

Send and receive signed and encrypted messages in Gmail with this Firefox add-on. Once installed, you'll notice a padlock icon when composing an email. Give it a click to lock and encrypt your email, and rest easy knowing your top secret message will stay that way.
Free, https://addons.mozilla.org/en-US/firefox/addon/592
Email This!

We like any tool or service that makes our lives easier and streamlines common tasks, and one such utility that fits the bill is the Email This! Firefox add-on. Once installed, Email This! will appear in your right-click context menu making easy work out of emailing the title, link, and any highlighted text through your Gmail account.
Free, https://addons.mozilla.org/en-US/firefox/addon/3102
Gmail Manager

This Firefox add-on is like a supercharged version of Gmail Notifier. Probably the best feature is the ability to add multiple accounts, but you can also configure how often Gmail Manager checks for new messages, display new mail snippets from the inbox, play a sound when new mail arrives, hide your email alias, and more.
Free, https://addons.mozilla.org/en-US/firefox/addon/1320