APL Wiki
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
IBM sells IBM APL2
Jsoftware sells J, an APL completely redesigned about 1990.
Kx Systems sells K and Q, blends of APL and Lisp.
MicroAPL sells APLX
NARS2000 a free Open Source Experimental APL Interpreter
See also WhichApl and AboutApl/AplsAndRelatedLanguages.