Today Matt Cutts posted a video on Negative SEO and the Google Disavow links tool.
With all the updates that Google has made this year with panda and penguin there have been many concerned with negative seo. Negative SEO is where a competitior may place links to your site that are totally unrelated to the content of your website in the hopes that Google will think you did these links and lower your ranking or de-index your site completely.
Matt says most don't have anything to worry, if you are in a competitive niche use the Google disavow links tool.
Here is the video: