title_body: What are the different file formats that eBooks are available in and which of these is most widely supported?
upvoted_answer: There is an extensive (though maybe not exhaustive) list of ebook formats on Wikipedia. The article does a comparison on the formats.
  
  The most widely supported formats are epub and pdf (and mobi/prc / KF8 / AZW if you include Amazon Kindle). Of course, plain text is probably THE most supported format, but I'm assuming you meant something that supports special formatting etc.