MouseHole is a proxy that has the ability to rewrite webpages, just like GreaseMonkey, but instead of using JavaScript it uses Ruby.

Hoodwink.d is a comment system that is added by MouseHole to weblogs. It’s a really nice way to combine serverside, with clientside.

This combination create the possibility for people to add comments to all kinds of websites.

What other things could be done to by using technology like this?