Sumatra PDF supports viewing CHM files. Sumatra is free, open-source and supports many other formats, like PDF, ePub, MOBI, XPS, DjVu, CBR, CBZ.

XCHM is a cross-platform, open-source CHM viewer. It works on Windows, Mac and Linux.

FBReader is a cross-platform, open-source eBook reader that can also read CHM files.

CHM Reader is not a stand-alone application but an extension to FireFox which allows reading CHM files directly in that browser.

ExtraCHM is a commercial CHM reader.

Beyond CHM is a commercial, multi-tab CHM reader and editor.

UltraCHM is a commercial CHM viewer.

CHM stands for Compiled HTML and is a format introduced by Microsoft to be used by Windows application to provide documentation.

CHM document is a collection of HTML pages, compressed and packed as a single file. Files are compressed with lzx algorithm. CHM document also contains additional files like pre-build search index, table of contents structure etc. For the most complete description of the CHM format, you can read Unofficial HTML Help Specification.