From Fedora Project Wiki
Easter Eggs are hidden messages in software programs. They are usually funny or quirky messages. Fedora has a number of them
Firefox
about:mozilla
about:robots
Reference:
http://en.wikipedia.org/wiki/The_Book_of_Mozilla
GNOME
Press alt+f2 and type "free the fish" as the command without quotes
Vim
- vim
- help 42