Mine is FrexxEd (free, open source). Powerful right out of the box, and insanely configurable through it's script language FPL which is 99% C (if you know C, then you know FPL). If some feature you need or want is missing, just write it yourself in FPL. The possibilities are almost endless.
FrexxEd here too... I'm working on an AROS port. Actually, I spent weeks improving the AROS console.device/handler because it infuriated me when I started working on the AROS port of FrexxEd...
The changes I made for AROS should almost all make it easier to port to OS4 too (though I hear the 68k version runs fine on OS4).
After that I plan on starting to add a few new features, update the look slightly etc...
