WireFly
Wirefly's site is very similar to most sites selling mobile phones online. You can browse by phone maunfacturer, carrier, or service plan (individual, family or prepaid), with each plan being categorized according to carrier.
They also sell a huge range of accessories for your mobile phone, and this part of the site is very easy to navigate. If there's an accessory you need, chances are they sell it, and because they let you select the accessory according to your specific phone, you can be assured it'll work with your exact model.
Finally, should you be in the mood to annoy all around you brighten up the sound of your phone, Wirefly also sell a wide range of ringtones. They even offer a feature called "U Doo: Do ur own ring tone", in which you can create your own ringtone, even more annoying than those on offer!