control+F4. Cycles through the applications in that Space.
(I actually have this mapped to control+`; which I find easier to remember, being close to the cmd+`; to cycle through an application's windows.)
To change the keyboard shortcut, go to the Keyboard preferences, Keyboard Shortcuts, and Keyboard & Text Input. It's the 'Move focus to active or next window' option.
(Edit to fix the keyboard shortcuts; I didn't realise that the backtick character wasn't showing up.)
