Google Alerts is an excellent free resource that in my opinion far to few people use. In short, it sends you alerts anytime your chosen search phrase is mentioned. Meaning, you can tell Google to alert you via email anytime your website is mentioned by another source.
To setup an alert visit: http://www.google.com/alerts
For a very good write with detailed instructions on how to use Google Alerts Check out this post in the SEOmoz blog: http://www.seomoz.org/blog/how-to-use-google-alerts-for-quick-and-easy-domain-alerts-10098
Update: Here is a new write-up from Sitepoint on using Google Alerts that is also worth checking out. http://www.sitepoint.com/blogs/2010/09/07/how-to-use-google-alerts/