Here are all of the useful (and not so useful, but still funny) about: triggers you can type into your address bar.
about: — The same page as “Help -> About”.
about:blank — A blank page. Useful for setting as your homepage.
about:cache — Displays cache statistics and disk cache directory location.

about:config — GUI for modifying user preferences.
about:crashes — List of Breakpad crash reports, with links and datestamps. Only in products based on Gecko 1.9 or later (Firefox 3, Thunderbird 3, SeaMonkey 2).

about:credits [...]

For those of you playing with Firefox 3 – Beta 5, try typing about:robots into your address bar and hit enter.

So what does Klaatu Barada Nikto mean? Shame on you – this is one any geek should know…