That would be Ctrl + 3 (though not as well known as Ctrl + 1).
What it does: it opens a menu where you can type and it takes all text from buttons, commands, menus etc. and tries to match what you just typed to it.
In a typically Eclipse way, typing "Ant" matches
1) ANT from View menu as well as
2) Extract ConstANT and
3) Create A New Target definition
Emphasis and caps - mine.
It's a "Quick Access" shortcut, so it's a shortcut to many, many things - for me, king of shortcuts then.
For all who would want a different king of shortcuts - what would it be? ;-)