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Sumatra PDF supports viewing mobi ebooks Sumatra is free, open-source and supports many other formats, like PDF, ePub, XPS, DjVu, CBR, CBZ.
FBReader is a cross-platform, open-source eBook reader that can also read mobi files.
Cool Reader supports mobi and a few other formats.
Calibre is an eBook library manager with built-in eBook viewer.
Details of mobi format
Mobi format was invented by French company MobiPocket as an ebook format for their MobiPocket Reader application. The company was bought by Amazon and the format was used in Amazon's Kindle. Unfortunately Amazon added DRM to the format, which means that only Amazon's software or devices are able to read ebooks purchased from Amazon.
Mobi format is a simplified and adapted HTML. There is no official documentation of the format, but it has been reverse-engineered and documented by the citizens of the internet.
Mobi files usually have .mobi extension. Amazon's eBooks have .azw extension. Because the format was originally designed for Palm Pilot PDAs, they can also have .prc or .pdb extension.
Free mobi books
List of websites that offer free ePub/mobi eBooks for download.