Cadence BPM Tapper is a free application that takes as input a user's clicks in time with the beat of a song and exports the click rate as beats per minute to the metadata of the currently playing song in iTunes. It serves as a form of publicity for RehabTechnologies' commercial line of Cadence music analysis and streaming applications, which can analyze the BPM of all the songs in your library on its own. However, for those unsatisfied with automated BPM analysis in general, who prefer a more manual approach, BPM Tapper provides a (relatively labor-intensive) alternative.
There is not a huge amount to tell about BPM Tapper because, in fulfilling its simple function, the app has a very simple interface. Clicking the big white button in time with the beat of the song playing in iTunes generates a BPM number; clicking the number exports the value to the song's metadata. If there's a preexisting value, it is displayed in smaller text above the beat you're tapping. Irregular (e.g. double-tapped) beats skew the number, but after a small number it forgets the taps that went before.