If you're trying to put together a playlist that contains songs that are a similar speed, knowing the beats-per-minute of each song is immensely helpful. BPM Analyzer from MixMeister that will analyze all of your MP3 tracks, and update the ID3 tags in them with exact BPM information. With that done, it's a simple matter of sorting your music library by the beats-per-minute field.
Now you can put together an upbeatplaylist full of fast songs, a mellow playlist with slower songs, or pretty much anything you desire. It's great for working out or jogging, trying to find the perfect beat to keep your pace.
BPM Analyzer is a free application available for both Windows and Mac.
If you're trying to put together a playlist that contains songs that are a similar speed, knowing the beats-per-minute of each song is immensely helpful. 
