REXX is the IBM SAA (Systems Application Architecture) standard, user-friendly programming language. It is available for IBM mainframes, Unix, the Amiga, DOS (Quercus's Personal REXX), Windows, and other platforms. It has been a part of standard OS/2 since Version 1.3. Programs written in REXX that do not use system-specific libraries are fully portable.
OS/2 2.x comes with an online REXX reference, and printed REXX documentation is available (Mike Cowlishaw's REXX book, IBM's twin guides). The Usenet group comp.lang.rexx discusses REXX programming.
It should be stressed, however, that OS/2's online REXX reference is a reference, not really a tutor. Paul Prescod (papresco@undergrad.math.uwaterloo.ca) is working on a free REXX tutor, which will be available on cdrom.com.
Watcom also makes VX REXX and Hockware makes VisPro/REXX. Both of these products allow quick and easy development of REXX PM programs.