Please do correct me if I'm wrong, but I've just recently researched APL and it seems like beyond J there really isn't anything else. There's a lot of recent research however, so maybe soon you'll see something new, but in general array based languages seem very little used and studied.
proprietary: k7, k2/k3 (very frequently used so including them separately), dyalog APL, APL2 (still shipping), APL2000, D
not proprietary: NARS, ngn/k, J, klong, aplette, gnu apl, kona
javascript: ngn/apl, oK
calculator (free software): ivy
dead but the body's still warm (free software): xxl, nial,
dead but the body's still warm (proprietary): aplx, a+
There's a bunch missing here because everyone alive has written an APL interpreter at some point (Ken Thompson did for example but it's ancient history and has a few descendents), but listing all of them would take hours. Especially if we're getting into historical stuff.
They've been pretty heavily studied, though, certainly:
I don't really see it as still alive: half of the commits in that repo in total were on that day, and only one was a change of code. Before it, the last code change was two years prior.
Correction now that I'm not on my phone: A+ was GPLed in jan 2001, making this 19 years ago - and indeed, the last change was in 2008 for 64-bit support, but at that point that project had been already quite stagnant for 5-6 years -- changes and releases were all concerned with documentation and newer build environments.
The "electric" GUI concept (The "+" in "A+" - without the GUI it was just called "A") was much simplified in k2 - it was fast, simple and extremely effective even though it didn't really look good - it is, in fact, the only environment I've ever had a chance to use where having a GUI is as simple or simpler than writing a batch program -- see e.g. 15-Puzzle[0] or the spring simulation[1] (even though the canonical example is the S- spreadsheet which you can find at the same site).
