I've been happy using BHphotovideo.com , Ritzcamera.com , and newegg.com ( although newegg is not limited to cameras )
I find it pleasant dealing with B&H since I live quite close to the actual store, but their online site is just as expansive as the store itself. I've heard some bad things about the quality of their packaging, though I've never gotten anything from them beat up.
I've bought things from Ritz and used their 'we replace it no matter what ' warranty nice, worked lovely. However, it was a brick and mortar store, not online