A friend came over and listened to the speakers that sit on my desk. They're pretty good, but he felt they could be improved by isolating them from the desk. So we put each speaker on a huge phone book and used some computer manuals to tip them back a little. The result was a definite improvement in sound quality. The highs were a little less strident. The bass was somewhat tighter. It was not an overwhelming difference, but it was easily perceived.
I thought perhaps we could do somewhat better with proper foam. I found various people talking about Auralex's MoPads on the web. They waxed poetic as to their ability to isolate the sound. Frankly the price at around $30 was the thing that sold me. I found a pair on eBay at a slightly reduced price (including shipping) and bought them.
After replacing the phone books with the MoPads I'd say that the sound is roughly the same. If there is a difference it's almost imperceptible. So If you want to isolate your speakers really cheaply I'd say use phone books and heavy manuals to achieve the desired angle. But for a relatively small amount of money you can get the right tool for the job. I'm not aware of any other solutions to this problem, so I'd have to say this is the answer for isolating speakers or anything else that might vibrate or resonate.
Overall, the tuning of a room is often to most overlooked area in achieving good sound quality. It certainly is the area that I have ignored knowing full well that I could get quite a bit of qualitative improvement with some high quality foam. I suspect that these are the first step for improving the vast majority of rooms. They should also work well for isolating speakers from speaker stands. I suspect speaker stands filled with sand wouldn't require such treatment, but many others would probably benefit.