McAfee Site Advisor is a commercially run service run by one of the world's leading security companies. If you want the highest level of trust look here. The downside to their service is they are slower to index sites than their crowd-sourced competitor, WOT.
Web Of Trust is much fast, because it is crowd sourced. This means regular internet users rate websites. Keep this in mind when reviewing it's ratings. If a large number of raters have rated the site, you can be more confident it it's recommendation.
Webutation is an aggregator, meaning they combine results from various sources.
Google Safe Browsing is a very simple and free service available to anyone with a browser. To view the safebrowsing status of a web page, simply put this url into your browser, replacing www.healthypasswords.com with the website you really want to check. http://www.google.com/safebrowsing/diagnostic?site=http://www.healthypasswords.com/
If you already use Google Chrome or Firefox as browsers, your browser automatically checks the database before going to every page you request already, so you don't need to check this separately.