Sometimes all they need is a really good cleaning with something like TSP. And if you want to paint them, that's not very hard to do.
Check out some of these ideas:
To get mine to look new I use Old English furniture polish on them. It is good for the wood and cleans and shines them at the same time. I do this twice a yr for fall and spring cleaning. You can find it where furniture polish is sold.
Glenna: Paint them! ...don't forget to prime first -- and remove hardware (for a professional look) ...then two coats of a GOOD latex paint. "New" kitchen!!! ...and you can simply do it one section at a time
Replacing the old handles and knobs with new or different colored ones is an easy way to add life to old cabinets.
Ebay often has better-than-retail prices on bulk purchases.