That is Cmd-` (~ would require a Shift)

And after Cmd-Tab, Cmd-~ takes you backwards in the list of apps. So if you Cmd-Tab too many times, Cmd-~ takes you back instead of having to cycle through.

Likewise, in Safari Cmd-} (aka Cmd-Shift-]) and Cmd-{ take you forward and backward through the tabs. This also works in Terminal.

Cmd-L puts the focus on the location bar in Safari if you want to type in a URL (or tab over to the search)

Quicksilver is cool, but I just use Spotlight for launching apps. Cmd-Space turns on Spotlight.

If you use TextMate, I highly recommend the TextMate book from Prag Press.

Those are some of my favorites.