.com,

Comics have a “special” set of extensions to designate them as “comics”. CBR, CBZ and TAR files are comic archive files. A CBR file is the same as an RAR file, CBZ the same as a ZIP file, and CBT the same as a TAR file. The designations allow computers to associate comic reading software with files and allow other non-comic products to use the standard extensions.

These are free software image viewing programs designed specifically for reading digital comic book files, particularly those in .cbr and .cbz formats (and .rar and .zip formats, if they haven’t had their file extensions changed to .cbr and .cbz yet).

Windows:

* CDisplay, the Windows Sequential Image Viewer for Windows by David Dayton.
Available here: http://www.geocities.com/davidayton/CDisplay.html

* pixelComic, a skinnable comic book viewer written in C++.
Available here: http://fileforum.betanews.com/detail/pixelComic/1058250114/1

* CBViewer
Available here: http://sourceforge.net/projects/cbviewer

Linux:

* cbrPager: a simple comic pager for Linux (using GTK+)
Available here: http://www.jcoppens.com/soft/cbrpager/index.en.php

* QComicBook (using Qt)
Available here: http://freshmeat.net/projects/qcomicbook/

* Comical, the UNIX, Linux and MacOS X Sequential Image Viewer (using wxWidgets)
Available here: http://www.sketchyorigins.com/comics/forumdisplay.php?f=47

* Asparagino’s Comic Viewer
Available here: http://home.asparagine.net/software/comicviewer/

Mac OSX:

* FFView
Available here: http://www.feedface.com/projects/ffview.html

* Comic Book Viewer
Available here: http://gumby.misplacedmac.com/

* Jomic (written in Java, so should be cross-platform as well)
Availabe here: http://jomic.sourceforge.net/