You can open and view Windows Compiled HTML Help (CHM) files easily with CHMOX, which is a .chm file viewer for Mac. Chmox is a 100% native Cocoa application to read CHM (compiled HTML) ebooks on Mac OS X.
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Jake says
There are a few freebie tools that read .chm I have Chmox.
Zipeg (sheep icon) will extract the embedded html file structure from a .chm.