nuffink has released ChordSpace, a free VSTi plug-in for Windows that generates MIDI chords. No knowledge of theory is needed to use it; you simply click around until you find a progression that works for you. ChordSpace has a palette of over 200 chords per key, all logically laid out so that great progressions are easy.
If you would like to understand a bit more of the theory behind it the ChordSpace website has a friendly overview.
