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.
xCHM at Ohloh.
xCHM at the GNU directory.
xCHM at Freshmeat.
The Mac binary at MacUpdate.
A satisfied customer.
Another satisfied customer.
Yet another one (Linux build tips included).
Lots of people like xCHM.
Chmtools.com lists xCHM.
Helpware lists xCHM.
The 'OSX Killer Apps' page lists xCHM.
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.
Another user blogs about xCHM.