Here is a summary of REX addin development options, current as of August, 2001. Click the links in the paragraphs below to got to the homepage of for each option:

Z80 assembler Hard work, but the most efficient way to code, particularly in view of the limited addin size and if you already know Z80 this might appeal to you.

The rexdk - THIS IS THE BEST CURRENT OPTION.
The rexdk is a modified version of the z88dk. Support for the REX was developed by Daniel, Dominic and Damjan.
Also this can be combined with Shai's Borland tools and free Turbo C++ 1.0. Now you have a source editor, compiling your files with one keystroke and best of all the editor automaticaly jumps into compilation error locations by clicking on the apropriate error message.

Rex sdk. The rex sdk by Mike Ludwig. A new option that contains the following programs: - ANSI C compiler - Z80 assembler - Object Linker - Icon Creation Utility - Rex to Emulator Utility
Source written for rexdk should be able to be compiled under this SDK with little modification.

The DS2 SDK : The DS2 SDK although itself free requires a commerical C compiler ($200) to operate. Dimitriy has purchased the compiler and set up a telnet and ftp server so others can test the compiler. The legality of this may be marginal, and its basically for testing only.

Small C SDK This is not as powerful or efficient as rexdk. The variable handling compiles rather inefficiently and the type support is more limited than rexdk. Further development is on hold.

The RxSDK (Based on the SDCC - Small Device C-Compiler). This was adapted for the Rex by Marc Nijweide. Its in an early stage, and not yet a choice for serious development