ZeroServ is a tiny web server (60k) which purpose is to block any incoming advertisements by blocking the host and allowing your browser to configure to it via proxy configuration. There are multiple ways to block the ads these days, either on the browser level or one step up as ZeroServ does. This method would work for any browser that can support automatic proxy configurations.
It’s not open-source,but it’s freeware and source is included.
Via Ranish