Handy when you need it and frustrating when you forget how you accidentally turned it on: That's XWindows' keypad mouse mode.
It is actived by pressing "Num Lock" while holding down the Shift key. Hit Shift + NumLock again to toggle out of the mode.
The numbers around the edge (7,8,9,6,3,2,1,4) move the cursor up, down, left, right, and diagonally. 5 and Enter become left-click; 0 becomes right-click.