Get New Mail: Cmd-Shift-N
If you're waiting for a message, this can be really handy. Cmd-Tab to Mail, hit Cmd-Shift-N, then Cmd-Tab back to your previous app.
Reply: Cmd-R and Forward: Cmd-Shift-F
These pretty much go together. Cmd-R will reply to your message and Cmd-Shift-F will forward your message. Always handy if you have a lot of messages to reply to.
Mark as Junk Mail: Cmd-Shift-J
No matter how hard you try, you will probably never be completely free of Junk mail. However, if you mark a junk message as such, then Mail will eventually learn what you want and what you don't want.