Tangerine

I was pleased to discover Tangerine, a Mac app that analyzes the songs in your iTunes library and determines their tempo and "intensity." Then it can automatically create random playlists that group your songs by these values, so you can easily make uptempo workout mixes or mellow chill-out mixes. I've been looking for something like this for a long time, and it works great. It exports the playlists you make back into iTunes. It doesn't export the BPM data, which I wish it would do, but even without that feature I'd consider this a must-have application for Mac iTunes users.