I'm somewhat fond of Amazon.com although I admit I have yet to purchase a book online.

What I dig about the site are the online add-ons: the search capabilities, the personal recommendation centre, the 'other people who bought books by this author also bought...' links, and the ability to read online reviews placed by both established critics and those customer comments from the everyman.

(the above is a a link to a Family Circus book which was reviewed quite humorously by faux-academics. Unfortunately, Amazon wised up and wiped most of them out although some have returned. Your mileage may vary.)