A very interesting new startup, CloudFlare, provides a cloud based firewall solution, of sorts. The way it works is pretty straight forward, you move your DNS to them, they in turn direct your traffic at their servers which cache your site and make decisions about the connections. By leveraging Project Honey Pot information they can deflect bad guys. The nice thing is that its almost entirely automatic. Via their dashboard you can get more explicit.
For the time being the service is free. It only takes about 5 minutes to get completely setup, so if your looking for something new and interesting to play with give it a go. At the least, read their story. Very interesting stuff.