1) Feedback Loop: Users submit URLs and ask for feedback on them. Each time they provide a feedback for an other submission, they get to view a feedback provided on theirs (hence the loop).
You can fine tune the system by having feedback receivers rate the feedback they've recieved (better feedback = more points for the person who wrote it = they get to see more feedback on their post or their post gets pushed to top).
I think it's a beautiful and self-enforced "you scratch my back, I'll scratch yours" idea.
2) Graffiti: This is a bookmarklet that allows a user to draw on top of a webpage (probably requires transparent flash). Of course, they can view other people's graffitis as well.
This would be pretty awesome, and has a lot of game aspects to it that would make it extremely addictive. For example, you could notify a user if anybody writes over their graffiti (eg, starting turf wars), and you could keep a record board for "most pages graffiti'd" and "most area covered", etc.
I feel like there's a very real possibility this could go viral and stay popular. And, I've got an awesome domain name for it. E-mail me if you're interested.