If your chairs have drop-in seats, consider a DIY reupholstery job with mismatched (but complementary) fabrics. You can always have a pro do a fancier tufted version (like the stools in the second photo) if you want something more formal.
Painting your matching chairs different colors is another idea. It might seem like a big commitment, but of course you can always re-paint them to match if you hate the result.
Or, if you've got some matching chairs and a few beloved singletons, and if the rest of your room is really, really simple, you could paint the chairs different colors and throw in the mis-matches. Hey, it works in the fifth photo, don't you think?
Has anyone out there experienced matching chair fatigue and done a DIY makeover? How did it turn out?