Vendors

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

IBM sells APL2
APL2000 offers subscriptions for APL+Win, APL+Linux, APL+Unix and VisualAPL
MicroAPL sells APLX
Dyalog sells Dyalog APL
Soliton sells SHARP APL (SAX on Unix)

Jsoftware sells J, an APL completely redesigned about 1990.

Kx Systems sells K and Q, blends of APL and Lisp.