iTunes 4 added a "BPM" (Beats Per Minute) field to its ID3 tag choices. Of course CDDB and it's ilk don't include BPM in their information so you have to fill this in yourself. The standard solution is to use a program which has you tap (the mouse or keyboard) to the beat of a song and displays the BPM of your tapping. Assuming you're not totally rhythmically impaired, the BPM of your tapping should match the the BPM of the song. Here's a JavaScript Beats per minute Calculator which you can use at this page or download the source to run locally off your hard drive. Since adding BPM is something you may want to do in iTunes now, someone wrote an AppleScript to add a BPM calculator to iTunes. I haven't tried it yet but it seems like a good idea.