KeyFinder is an open source key detection tool, for DJs interested in harmonic and tonal mixing. Designed primarily for electronic and dance music, it is highly configurable and can be applied to many genres. It supports a huge range of codecs thanks to LibAV, and writes to metadata tags using TagLib.
It's intended to be very focused: no library management, no track suggestions, no media player. Just a fast, efficient workflow tool.
The accuracy of the algorithm is pretty good. Here's a comparison with some other packages. A hundred tracks is not enough to draw any significant conclusions, and I don't claim that KeyFinder is better than other software, but I think it shows that the algorithm is in the same league.
The standard KeyFinder interface performs batch jobs, detecting the keys of a group of files quickly and simply. There is also a detailed analysis interface, which can be used to visualise chord structures, melodies and key changes in a single recording.