KeyFinder is an open source key detection tool, for DJs interested in harmonic and tonal mixing. It’s intended to be very focused: no library management, no track suggestions, no media player. Just a fast, efficient workflow tool. It supports a huge range of codecs thanks to LibAV, and writes to metadata tags using TagLib.
The accuracy of the algorithm is pretty good. Here’s a comparison with some other packages. I don’t claim that KeyFinder is better than other key estimation software, but it shows that the algorithm is in the same league as even the best paid alternatives.