Mail Act-On is a must have plug-in for all of you Mail.app users out there. With it, you can add custom keystrokes to rules you create within Mail.app by following the Mail Act-On naming convention of naming the rule according to the
CTRL key you want to trigger the rule.
For example, say you want to move an email to a folder called
grade 11, you create a rule to move email to that folder and you would name that rule
Act-On: 1 | Move to Grade 11. Hitting
CTRL+1 on the keyboard with the email you want to move highlighted will trigger the rule you have created. This may sound trivial, but for a power user, this can greatly increase efficiency.
Check out the application here.