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.
List of websites that offer free ePub/mobi eBooks for download.
A list of ePub format readers.
A list of CHM format readers.