[beagle3]

The 32 bit version of k4/q is free from Kx (the original K people) is now free from http://kx.com

There's a GPL implementation of K3 called Kona at https://github.com/kevinlawler/kona ; kevin lawler also has a language called kerf (not open source) that is based on the same principles with a different syntax (JSON+SQL+more).

John Earnest has a Javascript implementation of K5 called oK at https://github.com/JohnEarnest/oK

Andrey Zholos has a JITing implementation of a K-like language at https://github.com/zholos/kuc