I’ve come across a site called SiteAdvisor which is the product of a research effort at MIT. It aims to provide a protection from sites that contain spyware and/or are known to send spam emails. This is done by means of a browser plugin — available for both Firefox and <abbr title=’Microsoft Internet Explorer’>IE</abbr> — which has a green/amber/red safety indicator to inform you about the safety of a site. It tells you both about sites that you visit directly, and it will categorise Google and Yahoo search results, allowing you to avoid the dodgy sites.
It is in Beta at the moment, but it looks like it’s worth trying out.