Here is a list of places you can go to learn more and do more with Gmail:
Gmail Hacks/Tips by Cyberknowledge (http://cyber-knowledge.net/blog/2006/09/05/gmail-hackstips/) - Covers:
Gmail as a blog database
How to encrypt your entire Gmail session, not just the login
Create a Gmail partition to use all that space they give you
Install a Linux file system on Gmail
Gmail as a notepad
Change Gmail skins
Firefox plug-in to manage multiple Gmail accounts
gNotify - "You've got mail"
gDisk for Mac
GTDMail: integrate GTD process with Gmail
Gmail Tips - The Complete Collection, by Jim Barr: There is no way I am listing all of these, there are over 50 tips. Here are a couple:
Gmail Tip #40: New Feature! Rich formatting!
Gmail Tip #41: How Much Space Do I Have?
Gmail Tip #42: A More Worldly View
Gmail Tip #43: Notes in Just Two Clicks
Gmail Tip #44: Using An Email Client
Gmail Tip #45: Backing Up Your Gmail Account
Gmail Tip #46: Creating a Map Repository
Gmail Tip #47: Gmail Adds Chat!
Gmail Tip #48: Reply Without Scrolling
Gmail Tip #49: Moving the Chat Quick Contacts Box
Gmail Tip #50: Changing Subject Text When Replying or Forwarding
Gmail Tip #51: Keyboard Shortcuts
Gmail Tip #52: Can I "Delete all" Spam?
Gmail Tip #53: How Can I View Outlook Stationery In Gmail?
Gmail Tip #54: New Feature! Select ALL conversations
Gmail Tip #55: Excluding Chat from Search Results
You knew Lifehacker was here somewhere; Gmail Tips and Tricks - Monster Roundup by Adam Pash: Video on setup Setting up labels Setting up filters Setting up multiple accounts Shortcuts Searching Taking it on the road