It's unclear, since this case is basically making all software illegal in retrospect. Browsers freely implement each others' APIs; SQL servers all use the same API developed by a myriad of entities; the Unix API has been copied over and over and it's impossible to determine ownership. This was a really, really, really bad decision, and it will get worse if Google loses the fair use case.

If you thought software patents were bad, you ain't seen nothing yet.

> What are the implications for my free, open source software? And my closed source commercial software?

Software that implements other party's APIs might be obligated to agree to a licensing agreement - projects like Wine, Samba and various 3rd-party emulators. Ironically, Microsoft's discontinued, Android-compatible project Astoria would be exposed (to both Google and Oracle) - Microsoft submitted an amicus in the first Oracle v Google case supporting Oracle's cause. I am not sure if Xamarin- and Cordova-/Phonegap-type projects would be subject to Apple's lawsuits.

I also suspect web-APIs might also become risky, making it possibly for Amazon to sue you for providing AWS-compatible APIs[1] for example

Disclamer: I am not a lawyer, this is just my layman's perspective.

1. https://en.wikipedia.org/wiki/Eucalyptus_(software)

IANAL, but my take: Are you re-implementing anybody else's API without their permission? If not, this has absolutely no bearing on you. Hopefully a real lawyer can shed more light.