Read CHM is described as 'is ultimate CHM (Compiled HTML Help) file reader'. There are six alternatives to Read CHM for a variety of platforms, including Mac, Linux, Windows, iPhone and BSD. The best alternative is xchm, which is both free and Open Source. Other great apps like Read CHM are Clearview X (Paid), Kchmviewer (Free, Open Source), CHM Reader Pro (Free) and Enolsoft CHM View (Paid).
Kchmviewer is a free, open-source CHM (MS HTML help) and EPUB viewer written in C++ for Unix, Mac and Windows systems. Unlike most existing CHM viewers for Unix, it uses Trolltech Qt widget library, and could optionally be compiled for better KDE integration.