About 10 years ago, A+ was just used as the communications protocol for fixed income. Nothing (that I knew of) used it, but C++ ceremoniously created A+ objects, sent them over the wire to another C++ process that decoded them.

Around that time, we moved much of the mortgage models over to kdb and q, which was fun. Lots of weird dark little corners in that language. (List of dicts? Nope! It's a table now! 'sum x' drop NAs, while 0+/x was NA if any value was NA, but maybe only for ints, not floats, etc. Ugh.)

It is still there and open source [0] and I guess you can say k (not kdb or q!) are it's successors in some ways. Definitely in the way they are all very much APLs, with or without the glyphs. If you read the APL book by Iverson and then go to implementations of APL, A+, J, K, you'll notice differences, but there are all identical enough to pick them up fast once you are proficient at one of them. kdb+/q are different beasts and that is were the issues start (and the big bucks are to be found); debugging (performance) issues related to critical software written on top of k.

[0] http://www.aplusdev.org/index.html