J[1], ngn/k[2], oK[3], kona[4], and GNU APL[5] are the big oss interpreters. The main closed-source ones are Dyalog APL, Kx k, and Shakti k (which are still free for non-commercial use).

1: https://github.com/jsoftware/

2: https://bitbucket.org/ngn/k/src/master/

3: https://github.com/JohnEarnest/ok

4: https://github.com/kevinlawler/kona

5: https://www.gnu.org/software/apl/