If you're an aspiring DJ or are holding a party, then it's useful to know the beats per minute (BPM) of your tracks so that you can mix them in a way that doesn't sound terrible.
BPMer is a plug-in for iTunes which allows you to do exactly this, so that you can better mix your tracks. BPMer features a completely automatic BPM detector which can process batches of MP3 files or entire iTunes playlists. It can even automatically write the BPM to the metadata of the MP3 file and export the results to iTunes, so glancing through your playlist makes it easy to identify tunes with a similar BPM rating.
While for most MP3 or AAC files BPMer works fine, it does sometimes struggle with certain tracks. If it can't detect the right BPM, it applies a random number, so be careful when mixing for the first time! The other problem is that it does take quite a while to analyze your music. The same goes for showing the progress log, which almost crashed my system.
The big advantage of BPMer is that you don't need to put your music in an external application to calculate their BPM - you can do it directly in iTunes. The disadvantage, as mentioned, is that results may not be entirely accurate and it can be slow.
Note that to see the BPM in your iTunes playlists, you have to select 'View' in iTunes and then 'View Option', and select 'BPM'.
BPMer has plenty of bugs and drawbacks but it is one of the most convenient ways to assess the BPM of your iTunes collection to make your mixes sound more impressive.