The author of Nial, Mike Jenkins, has recently released v7 of Nial. Nial is akin to Q in that many of the operators are keywords rather than symbols. Its computational model is slightly different due to its roots in Trenchard More's array theory.
For a certain set of applications, I believe the difference is speed. K has been said to even outpace C, even though that's the language it's written in. AW did an amazing job with his optimizations.
k variants:
kona (C, interpreter): https://github.com/kevinlawler/kona
klong (C, interpreter): http://t3x.org/klong/
kuc (C++, JITted): http://althenia.net/kuc
oK (JS, interpreter): https://github.com/JohnEarnest/ok (see also iKe by John Earnest)
cousins:
J (C, interpreter) http://jsoftware.com/
A+ (C, interpreter, unmaintained): http://www.aplusdev.org/index.html
Gnu APL (C, interpreter): https://www.gnu.org/software/apl/
apl.js (JS, interpreter): https://github.com/ngn/apl
There's also NARS2000 and a few other APL interpreters
related:
Numpy and R provide similar functionality, albeit with more verbose (and less fluent) composability. They are usually slower.