We (the TGHack list members) are writing a bunch of tools, to develop demos and new games for the PC-Engine. They're available from the TGHack-ftp site:
ftp://joyce.eng.yale.edu/pub/TGHack
This is an incomplete list, of what we have so far:
A simple (but useful) Disassembler understanding HuC6280 Codes. Written by Joseph LoCicero,IV and Dave Shadoff
This is a simple MML-compiler written by me (Jens Ch. Restemeier). The CD-ROM system cards have a build-in music player. A small compiler is necessary, because it expects binary data.
I found a bunch of floating point functions on the net. They're designed for Apple ][, but I think we can use them, too. Look at:
http://sls.mcs.usu.edu/~kurto/lynx/programming/index.html
There are a few other tools in development, for example a graphics-converter and several assemblers.