Gmail is a great email service and we have been writing several Gmail Tips here, however no matter how many tips and tricks we right, there are always going to be quite a few things that you can still learn.
Google has now created a new page where they list out tips and tricks for Gmail Ninjas :-).
The tips are divided into 4 different categories, White Belt for people who receive only few emails, Green belt for people who get dozen or so messages a day.
Black belt for people who get a lot of messages a day, and Gmail master for people who receive massive amounts of email message a day.
So You Want to Be a Gmail Ninja? [Official Gmail Blog]