I really like Site Advisor, the best thing about it is the search engine integration, check out this screen-shot*
As you can see, you get a quick report before you even click on the link, only visiting green sites mean less pop-ups, less spy-ware and in overall a better Internet experience.
So Why am I mentioning this now ? Well my site just got it's green tick, I've also signed up as a reviewer. Being a reviewer means that you can add the 'human factor' to their tests, so If you get a bad experience from a green site, or find a miss classified red one, you can get your voice heard !
The plug-in works in both windows & linux (I've tested both) and doesn't interfere with normal operation, so it's defiantly worth a download !
*Screen-shot from Firefox with google preview installed. Note, the coloured pop-ups only happen when you have your mouse pointer over them, so this pic is a mash of two !