Also
free @ Kobo, which likely means Borders too*. Available to Canadians.
* Sometimes Borders books show as non-free, even if you do the desktop app search, until you click on the non-free book listing for details, and then it turns out it's actually free on the in-app product page while still saying it's full price during the search results.
I must say that the Borders app experience has given me a new appreciation for the Sony Reader Library app.