After I found out about the power of shift-space in most web browsers, I went and played around a little more and I found a few more things I didn't know about.
While space in Mail.app will pages down in the message you're viewing (until you get to the end of the message, at which point it goes to the next message), shift-space will go to the previous message. Unfortunately, it doesn't actually page-up first, and instead it just jumps to the previous message.
In iTunes, Enter (as opposed to return) will put the title of the currently selected track into edit mode.
And I found this one out a few weeks ago, and I really love it: In many apps (including Mail, Safari, and iTunes), command-option-f will select the "Search" (on in the case of Safari, the Google) search widget.
It took me an embarrassingly long time to figure this one out, though, because I used to have command-option-f mapped to "skip forward 20 seconds in iTunes" using Key Xing (an excellent keyboard mapping tool). Which kind of makes me wonder if any of my other keymappings -- or even LaunchBar's command-space invocation -- are masking anything interesting.