APL Wiki
Which APL?
Where to start — and with which APL? There are several to choose from.
Self-starters
If you have no prior experience with an APL, no teacher and no textbook, download NARS2000, an Open Source APL interpreter, and make use of the excellent APL Tutorial on the APL wiki.
Classic APL
Availiability of textbooks for recent implementations of APL is limited. J, an array-based programming language with APL roots, is an honourable exception. The classic textbooks do not covert recently-implemented, important features of modern APLs such as object-oriented concepts, GUI (graphical user interface) implementation, program flow control structures, .NET interfaces, direct definition, namespaces and user-defined operators. But if you have a classic textbook, such as Gilman & Rose, you can use it to learn the fundamentals of the APL programming language.
The advanced features of APL+Win from APL2000, including ActiveX, GUI development and web services, have been fully documented with practical examples in the recent book Askoolum, A. "System Building with APL+Win" Wiley 2006, which is available from Amazon or directly from APL2000.
In this case, you will find the ISO-standard APL I-APL for Windows will serve you very well. You can download it here.
Open source
A+ is an APL originally developed by Arthur_Whitney for financial-markets applications at Morgan Stanley, an American investment bank, where it has been used since the mid-1980s, and subsequently released under GNU Public Licence at aplusdev.org. The BAA built a Windows version of A+, which you can download from the BAA A+ project.
Industrial strength
APL2, APL2000, APLX, Dyalog APL and SHARP APL are primarily used for commercial programming and run on a variety of PC and mainframe platforms. They are extensively documented but offer little material to support new programmers. They are shipped with many features to assist working programmers and are excellent tools for general programming.
Visual APL is an object-oriented, Unicode-based APL implementation which is fully-integrated with Microsoft_Visual_Studio and produces fully-managed code which inter-operates with any other .Net language, such as C# and VB.Net.
While licences for these interpreters typically cost several hundred pounds, some vendors (e.g. APL2000 [both APL+Win and VisualAPL], Dyalog, IBM and MicroAPL) offer licences for educational purposes either free or at large discount. MicroAPL and Soliton Associates offer free personal versions of (respectively) APLX and SHARP APL for Linux.
Q is the latest APL from Arthur Whitney, tuned for fast execution and very large database queries. It sees most use in financial markets, where server licences sell for tens of thousands of dollars. Educational licences are available for teaching; contact KX Systems.