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There's A+, from Morgan Stanley. It is actually a variant of APL, but it looks mostly like it (unlike J, it retains APL's hieroglyph characters, etc.) Licensed under GPL.

The A+ website is http://www.aplusdev.org/ for those interested.

FWIIW, I couldn't get this to build.

It is also in the Fedora repositories (yum install aplus-fsf). I would imagine that it would be in Debian/Ubuntu too, but I don't have a current install to check. If all else fails, you can probably grab the source RPM to see how Fedora/Redhat got it to build.

https://github.com/kevinlawler/kona is an open source implementation of K3.2

(Though note that the most recent version of K, K4, is a much cleaner and simpler language - I'm not aware of an open source version of that)

There's OpenAPL, but I've no experience with it (yet).