Most of you probably utilize all of these features, but just in case!
1. Better Gmail Firefox add-on
2. Create keywords
3. Experiment with Labs
4. Gspace
5. Gmail manager Firefox add-on
6. Step up your searches!
7. multiple inboxes
8. create Google docs right from email
9. filter via optional dots:

Optional Dots
I'll let you in on a little secret. You know those little dots in your Gmail address--like the one in 'john.doe@gmail.com'? Gmail doesn't recognize them as part of your email address. That's right, john.doe@gmail.com will receive all mail that's sent to johndoe@gmail.com, joh.n.doe@gmail.com, and even john...........doe@gmail.com or j.o.h.n.d.o.e@gmail.com. It's an interesting quirk of Gmail, and one that you can use to your advantage.

Suppose that you want an easy way to separate business e-mail from personal e-mail, without having to individually label each message. Simply give your business clients the e-mail address "john.doe@gmail.com," and give friends and family "johndoe@gmail.com." Messages sent to either address will come to your inbox, and you can set up a filter that automatically labels messages sent to "john.doe@gmail.com" as "business."

Wow, didn't think of filtering via #9. Click on the ginormous Gmail image to jump to the article.

Additionally, Google's Gmail Ninja page has tons of neat stuff, including printable cheat sheets. Love the cheat sheets. Find them here.