Clever of them to show a custom welcome message for Hacker News visitors. I love how it jumps you right into a demo app with no login or registration steps.

The functionality and usefulness of this service is fantastic, but I wonder how hard it would be for github itself to compete; they've shown that they can do simpler binary versioning and comparison with PNG/JPEG images, so all they'd have to do is add support for previewing PSD files as well. Whether that's a simple thing to support is not something I'm qualified to answer though :)