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, your best bet is J from Jsoftware. You can download it free on a variety of platforms, including the Windows, Macs, Linux and the PocketPC. It requires no special fonts or keyboard mappings, and comes with substantial tutorial materials and the J Wiki.
There are contemporary text books (see http://www.jsoftware.com/jwiki/Books) for both the language itself and its use in mathematics. The J Forum is active, and helps and encourages new J programmers.
Classic APL
There are no textbooks on the other modern APLs; J is an honourable exception. The classic textbooks tell nothing about important features of modern APLs such as object-oriented GUI support, 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 basics of the language.
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 a new APL specifically designed for use with Microsoft_Visual_Studio in multilingual applications.
While licences for these interpreters typically cost several hundred pounds, some vendors (eg Dyalog and IBM) 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.