Command + Tab App Switcher

Here's a simple one that's VERY handy.

You can use Command + Tab to bring up the Application Switcher and keep tapping tab to cycle to the right (including wrap-around). But here's the one that some people don't know about: once you have the app switcher up you can tap ~ (tilde) to cycle to the left with wrap-around.

Handy if you're doing more than quickly using Command + Tab to switch between two apps; if you're using three or more this is a very fast way to toggle between apps and easier on the fingers since some people use Command + Shift + Tab to cycle to the left.