If you use Serato DJ Pro, you might already own a harmonic mixing tool.
While the official Mixed In Key program remains a paid commercial software utility, open-source desktop applications like KeyFinder and free web engines like Tunebat provide Camelot-wheel compatible key detection without charging a fee.
However, if you are a working DJ who relies on perfect transitions and set-list energy, the official Mixed In Key software is an investment in your craft—much like a good pair of headphones or a reliable controller.