Alt+Space brings up the window command menu (or system menu), which allows you to move or resize windows with the arrow keys. Very few people seem to know about or use these features, which can cut way down on mouse usage. (Jon Galloway).
But how, you might ask, do you open the MDI system menu located directly beneath the normal system menu? Use Alt+Hyphen.
A nice background read from Jensen Harris. It seems they've choosen the Space because back in the Windows 3.1 days the icon on the top left of the window resembled the space bar on the keyboard. The MDI icon looks a lot like a hyphen. Although nowadays neither of these icons looks anything like a space bar or a hyphen, yet the keyboard shortcuts remain the same even in Windows Vista (nice comment from Kam Vedbrat).