Tapping in rythm with the song is not a good solution if you have four thousand files in your collection, but it can be useful in some situations.
Free online tool. Tap any key to the beat.
Is a free tempo detection app for iPhone designed to simply and elegantly get the job done.
It can show rounded and fractional tempo.
Free to try, $19.99 to buy (as of April '09).
A widget for your Mac OS Dashboard.
A tap-tempo device useful for estimating the BPM of music and setting the BPM meta-tags of the current track in iTunes.
Web site with tap-key mini application.