Our library uses Overdrive,
MyiLibrary, and
NetLibrary
With your [my city] Library card, ebooks and audio books are available through Myilibrary, OverDrive, and NetLibrary.
With Myilibrary and OverDrive, you do not need to set up your account at the library; you may register and begin downloading materials from anywhere that you have access to the Internet.
To access NetLibrary, you must first set up your account with NetLibrary at a [My City] Public Library computer.
Overdrive has the most titles, I think, but NetLibrary has better terms (10 out at a time, etc.)