This free browser plugin warns you about questionable links and threats via popups. Looks good and comes well-recommended from Chris Pirillo.
SiteAdvisor uses an easy-to-understand rating system to help Internet users stay safe as they search, browse and transact online. Among the many security threats and nuisances which SiteAdvisor’s ratings disclose and help prevent are spyware, adware, spam, viruses, browser-based attacks, phishing, online fraud and identity theft.
At the core of SiteAdvisor’s ratings is a massive proprietary database containing test results from millions of automated Web site visits, download installations, and e-mail registrations.
If you say, Google “music lyrics” with Site Advisor loaded, you’ll see unobtrusive icons next to each search result. Mouse over the icon to get a popup with a brief explanation, or click through to get full information. Safe sites have a light green icon, dangerous sites are a noticeable red.