I don't know about a web site, but if you're running a Mac, there's a program called Tangerine that will analyze your iTunes library and tell you each song's beats-per-minute. That way you can select a play list that has a bpm that matches the level of intensity you're looking for in your workout.

In general, you're looking for a beats per minute analyzer. There are some free packages for Windows, but I don't have experience with any of them. My experience with Tangerine is also limited, but it looked pretty good.

Hope that helps.

Don