UPDATE: You should read this before doing business with HeavyInk. I can no longer recommend this service.
Travis Corcoran, the founder of the very useful SmartFlix service, which rents instructional DVDs of all kinds, has started a "mashup of Amazon.com-for-comic-books." It's a great idea, and will have:
Facebook-style social networking (we'll likely support the OpenSocial API at some point), and a few other things.