FBReader is a free (and ad-free) multi-platform ebook reader.

Main Features:

• Supports popular ebook formats: ePub, fb2, mobi, rtf, html, plain text, and a lot of other formats.
• Provides access to popular network libraries that contain a large set of ebooks. Download books for free or for a fee. Add your own catalog.
• Highly customizable. Choose colors, fonts, page turning animations, dictionaries, bookmarks, etc. to make reading as convenient as you want.