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. We've got tons of things that no other comic book sites have: personalized RSS feeds, free shipping on every order (no matter how small), reviews on authors, artists and titles (not just issues), a recommendation engine, profile pages, "friends" lists, blurbs, mini-blogs, etc. We're going to have tons more features in the near future.
Shown here: Giant Teen Titans Annual 1967, which will be republished in December.
HeavyInk.com is now in public beta.