Many sites offer you free comic books. However, most of them have CBZ or CBR format, which requires you to have a software like Jomic so that you can read them on your computer.
JOMIC is a reader of digital comic books with CBZ, CBR and PDF format. It includes a two-page viewing mode that supports several image formats like PNG, JFIF / JPEG, TIFF and GIF. You can zoom in on a page, rotate it, display or hide the toolbar, the information about the comic book, change background color, etc. In brief, Jomic is a comic book viewer that is fully customizable. It is a freeware but requires Java version 1.4 or higher.