Google have recently launched a new tool to its Webmaster toolkit, giving you the ability to identify and remove (at least in Googles eyes) unnatural or spam inbound links (from other sites to yours).
Google have become more active in notifying webmasters of problems with inbound links, this now enables you to remove the offending link(s) from Google's link graph without contacting the owner of the other site.
You may have received a message from Google in the past saying that they have identified unnatural, spam or paid links to your website, this usually left you having to identify the potential problem links and approaching each website to get the links removed. Not a massively effective process and neither is it a good use of your time. Bing also launched a similar tool earlier this year.
Here is an explanation of this new tool from Googles, Matt Cutts
Steps to disavowing links
To ask Google to not take certain links into account when ranking your website, the steps are relatively simple:
Login to Google Webmaster Tools
Check inbound links through the "Links to your website" feature, download into a CSV to make the next steps easier