Spitfire Audio regularly release free instruments in their LABS series. The latest just dropped, and I'm downloading it as I type this. They're quirky, occasionally weird, but frequently inspiring. You need to install the Spitfire Audio app to get them, but it won't cost you anything.
And while you're at it, fill out their questionnaire and in two weeks' time the Spitfire Audio app will let you install the BBC Symphony Orchestra "Discover" package. An entire symphony orchestra in your DAW, for free. If you write stuff using MIDI instruments, this is an essential addition to your collection.