I keep linking to Amazon and Smashwords when I tell you about my books, because they’re the two sites I use to publish them. But Smashwords is an aggregator, and it makes my work available on other platforms. For instance, you can find books by Ian Pattinson on Barnes & Noble’s site if you have a Nook. (Garth Owen’s there as well.)
They’re available from Kobo as well. Thanks to Smashwords, you can find my books through Apple, Sony, Scribd and more, though membership and hardware restrictions make it hard for me to find direct links to them for you. If you get your ebooks from people other than Amazon or Smashwords, have a look for Ian Pattinson and Garth Owen. Their books could easily be on there.