Vendors

Different vendors provide different implementations of APL. (Your mileage will definitely vary.)

APL2000 offers subscriptions for APL+Win, APL+Linux, APL+Unix and VisualAPL
Dyalog sells Dyalog APL
GNU APL a free, Open Source, almost complete implementation of ISO standard 13751
IBM sells IBM APL2
Jsoftware gives away J, an APL completely redesigned about 1990.
Kx Systems sells K and Q, blends of APL (mostly) and Lisp (a bit).
NARS2000 a free Open Source Experimental APL Interpreter