Pabs' CHM specification page is the most complete unofficial one I've seen.
Pabs has created an IRC channel for talk about CHM and other help formats.
Apparently there's a Facebook xCHM page out there. While I'm honoured by the interest, I have nothing whatsoever to do with that page. I don't use Facebook.
Wikipedia page. Ohloh. Freecode (formerly Freshmeat). GNU directory.
Ted Leung talks about compiling and installing xCHM on his Mac OS X.
The 'Help authoring tools and output for Linux applications' article on the 'Technical Editors' Eyrie' site mentions xCHM.
Julius Huang compiled xCHM on Mac OS X Tiger 10.4.3 and provides a nice screenshot here.