FBReader is a popular (30+ million installs) ebook reader. Works on iOS, Android, Windows, Mac OS, Linux, and Chrome OS. FBReader supports many ebook formats, including ePub, mobi, fb2, HTML, RTF, plain text. The primary formats are ePub and fb2. FBReader uses its own book parser and rendering engine. The engine is lightweight, fast, and highly customisable. FBReader is not open-source since 2015.