This is cool. A few suggestions:

* Add a way to delete previews you've made.

* Add more viral hooks. FB wall post, email, etc.

* Do the same concept but with javascript.

* Load each website in a frame with a 50px bar at the top or bottom with your branding and more options. This is where you'll monetize. I know you're focused on usability right now, but if it goes viral you need it to pay you or it will go away.

* Don't bother with premium features until you have a working viral loop providing consistent growth.

Best of luck!!