If you've had some musical training and have an instrument to hand, working out a particular song's key shouldn't present too much of a challenge. But if you don't and/or haven't, there's SongKey.
Potentially of use to DJs and remixers, we suspect, this cross-platform VST/AU/RTAS plugin can analyse a polyphonic audio stream and works in real time. It displays three probable keys, assigning a 'Confidence' level to each one, and also has a Note Profile feature that shows you which notes are being played the most.
You can see it in action in the video above. SongKey is available now from the HoRNet website priced at €9.90.