I bought a cheap elliptical machine from Wal-Mart and I totally love it. I started running almost ten years ago, and my knees were starting to bother me from the constant pounding. I bought the elliptical machine last weekend and have been using it all week, and my knees are feeling better than ever.
First off, the elliptical is zero-impact, so it's a lot easier on my joints. Secondly, it exercises my legs more completely than running, so the muscles that support my knees are getting strengthened. Third, it provides a whole body workout for my abs, chest, shoulders, and arms, which running just can't do.
The only significant downside is that I don't get to feel the wind in my hair as I zip through the neighborhood... but on the other hand, I'm much less likely to get hit by a car. All in all, I'd highly recommend an elliptical trainer to anyone who runs -- your knees and hips and ankles will thank you.