https://www.literatureandlatte.com/scrivener/overview 

• $45 (but has cheaper educational pricing)
• Not just an ebook creation tool; full suite of tools for writers
• MacOS and PC

https://vellum.pub/

• $200
• MacOS only
• Creates both ebooks and print books

https://www.smashwords.com/

• Free
• Designed to allow anyone to publish, then sell
• DRM-free
• Integrated with public libraries' Overdrive system

http://pressbooks.com/

• Built on WordPress platform
• Professional publishing houses using it

Pressbooks is our favourite platform.  It's user-friendly, has lots of features, and will be useful to most people who want to create eBooks.  Highly recommended!

http://www.apple.com/ibooks-author/

• Viewable on Apple devices only
• Beautifully designed templates
• Drag-and-drop, WYSIWYG editing

Although beautifully designed and capable of producing stunning eBooks, the app isn't particularly useful in many contexts because it's limited to Apple devices.  Sorry, Kobo owners. :-(

If you'd like to convert files into an eBook-friendly format, use one of the following free online tools:

• ConvertFiles
  
• Zamzar
• 2epub
  

These sites offer no frills eBook-making.  You take a PDF or a MS Words file and convert it to an eBook format.  No design options are available. 

Once you've created or downloaded eBooks, you'll need to manage them.  

Calibre

• Free - open source
• Organize eLibrary
• Transfers files to your device
• Handles open (DRM-free) eBooks
• Converts file types
• Edits item metadata

• Free - proprietary
• Organize eLibrary
• Transfers files to your device
• Handles open (DRM-free) eBooks
• Handles DRM eBooks from vendors & lenders