[Where's an authority site for APL?]
http://chilton.com/~jimw/ is an excellent APL site, though it can be slow when a lot of people are hitting it.
http://www.rexswain.com/aplinfo.html is a decent site for apl links, but even though the page appears to be actively maintained, some of the links are no good.
http://izap.com/~sirlin/apl/ is a more "into the guts of things" sort of APL page.
http://www.nars2000.org/ has a portable, open-source version of apl2, if you'd like to play with it.
See also An APL playstation, Playing APL, A partial APL parser, APLish .