IBM offers a trial version of their Code/400 production. I'm not sure what the URL is for it, but I'm sure someone else will provide it.
As an FYI, there is a free RPG III and RPG IV editor located at http://www.rpgiv.com/codestudio
This editor is a 16-bit version for Windows, but works fine under Windows 95/98/NT.
It has full prompting, help text support, syntax highlighting, symantec AutoCorrection, etc.
It does not, however do syntax checking or compiling.