Amazon.ca is the Canadian arm of Amazon.com. The .ca site doesn't have all the bells 'n whistles the .com has, but it's still ahead of other book etailers in terms of features. Prices are competitive.
Chapters.indigo.ca is the result of an Indigo takeover of Chapters in early 2001. Huge inventory. They're the big boy on the Canadian online block when it comes to books.