CoolReader

Free open-source, XML/CSS-based e-book reader for Windows and Linux. There's also mobile version for Android and some eInk devices. Supported formats include FB2, TXT, RTF, DOC, TCR, HTML, EPUB, CHM, PDB, and MOBI. The program offers numerous settings for improved reading experience. It's arguably the best reader for Linux, but on Windows, it has some issues, such as linking e-book extensions to the program so it opens when clicking on a book file.