Scribd is deemed the newest challenger to Amazon. However this was pretty much discovered accidentally by publishers. One author - Bob Seidensticker, complained just a year ago that his book was being made available on Scribd.com (this is short-hand for scribbled by the way).
Scribd quickly took it down after the complaint was posted by publisher, Berrett-Koehler.
However one thing led to another & Scribd was later on discovered to be a very viable marketing venue for books.
Here's some background: Simply put, Scribd is like the Youtube for printed material. It's simple enough to understand - people get to upload material on to Scribd like presentations, files & yes... books too.
So how do we make money off this? Scribd is now uploading excerpt for books on to its site. It's quite similar to Amazon's "Search Inside" feature so it isn't that big a deal really. However publishers seem to be attracted to the idea of having a thousand more views than normal.
The combined material makes Scribd a proverbial library of excerpts for book lovers to enjoy. The lay-out is also pretty simple (in fact it's a knock off of Youtube so it's not that hard to figure out)
So there you go. A new marketing venue for your book. Scribd is FREE by the way so there's no cash out to join. They get a cut from books that are sold through their store though, but I bet you it's not as high as Amazon.
I've been to the site a lot of times because search engines usually lead me there when I'm looking for some items on literature, research material & most of the time non-fictional work. The pages are saved as text so imagine the depth of the content for your book as well as the categorization.
I don't think Scribd will beat Amazon, but I do think it's a worthwhile option to consider.