How do I read .CHM files in OS X? What software do I need to open and read a .chm document in OS X?
One of the best apps I've found is iChm. It has a Spotlight importer as well as the app itself.*  It's free as well as being open source.

Only issue I've had with it is that sometimes long "m dashes", like —, will not be present, and instead the two words will be run together. (Though, I've looked at the source code myself, and it appears to actually be a problem with libchm, the open source library that iChm relies on).

*NOTE: the version of iChm in the link above (1.4.2) does not have a Spotlight importer, but I've forked this project on GitHub and added a Spotlight importer in version 1.5. You can download 1.5 at http://www.markdouma.com/developer/iChm.zip. 

The GitHub project is at https://github.com/NSGod/ichm (it may be a day or so before I've pushed all of the changes).