I'll agree with pretty much everything Mr Frog said, but add a few things.
You can get some pretty interesting sounds when you fuse the key note and some of the aforementioned chords.
For example, if you play the key note (C for instance) and chord III (Em), you get 'C Major 7th' (C E G B) which is a nice, refreshing, almost 'jazzy' sound.
Personally, in a minor key, I quite like 'I m (II)' (Which is chord 1 w/ a minor 3rd, with the second note of the scale as the lowest note), so this could be, for instance, Cm (D) - (D, Eb, G, C).
You can find som cool chords by sitting at a piano, playing something as simple as a C chord, and then experimenting with adding notes, moving notes around and fiddling with unusual harmonies (eg, tritones, Aug 5's)
Have Fun!
Muz :D