Plug-ins for Winamp, iTunes, FS (5)

BPMer

Automatic-detection BPM counter.
Plug-in for iTunes / Mac OS.
Saves the BPM tag to the song file. Support various file formats (not AIF.)
Free to try, $19 to buy (as of April, '09).

BPMeter (a.k.a. BPM Meter) - Winamp

Automatic-detection BPM counter.
Stable.
Doesn't save to the BPM tag.
Can only analyze music while it is playing.
Freeware.

DJ Helper for Winamp 2.x

It's a mixer, but it contains BPM music analysis.
Does both automatic and manual detection of beats per minute.
Requires two Winamp sessions to work correctly.
Doesn't save to the BPM tag.
Can only analyze music while it is playing.
Freeware.

Record Box Editor for Final Scratch

Fully automatic BPM detection and mp3 file tagging.
Works only with FinalScratch
FinalScratch costs US$500.