Mobipocket (link: http://wwww.mobipocket.com) is an Amazon company (since April 2005) that makes Reader software called MobiPocket Reader and MOBI format eBooks MobiPocket books generally have a .prc or .mobi extension. It was founded in 2000 and is located in Paris.
MobiPocket also has a creator program that can be used to create content in this format. It is a free download from their web site and works on PCs only.
The MobiPocket web site has a very large list of eBooks in this format. This site is a convenient location to see what is available in this format. They are generally posted at publisher suggested list price. MobiPocket is also a distributor for these books so most (but not all) retail sites that have this format available got their eBooks from MobiPocket directly. Some of these sites may discount the price.