Google finally announced the launch of a Link Disavow tool for webmasters today, after months of anticipation. This is a tool that you can use to tell Google to ignore links if you feel they are hurting your search engine rankings.
You can see plenty more details about it here (along with a video from Matt Cutts).
One important thing to note about the tool, however, is that just because you do tell Google to ignore certain links, it is not a 100% guarantee that they will do so. It's more of a suggestion, and Google will still decide whether or not it wants to follow your instructions.
In a Q&A section in the official blog post announcing the tool, Google says:
This tool allows you to indicate to Google which links you would like to disavow, and Google will typically ignore those links. Much like with rel=”canonical”, this is a strong suggestion rather than a directive—Google reserves the right to trust our own judgment for corner cases, for example—but we will typically use that indication from you when we assess links.
It probably won't be an issue if you're using the tool the way it was intended to be used, but it's something to be aware of.