KeyFinder is described as 'open source key detection tool for DJs who are interested in harmonic and tonal mixing. Designed primarily for electronic and dance music, it is highly configurable and can be applied to many genres' and is an app in the Audio & Music category. There are four alternatives to KeyFinder for Windows, Mac and Online / Web-based. The best alternative is Mixed in Key. It's not free, so if you're looking for a free alternative, you could try Tunebat or tuneXplorer.
It provides you with song information for a massive library of music. It gives you key, bpm, and even values like how danceable or happy a song is. Also it can recommend stylistically compatible tracks that have similar bpms and keys which should mix well together.
What do you need to create perfect mix? No matter what method you use - harmonic mixing or beatmatching, you will need to know the basic characteristics of the audio track, such as the key and tempo. tuneXplorer will help you find the key of song and BPM value with high accuracy.