(Oct. 16th): Google launched the Disavow Links Tool

If your site lost rankings because of spammy links (in particular you got an "unnatural links" warning in the Google Webmaster Tools, was hit by a Penguin Algorithm update or are a victim of negative SEO), you can now mark those links as to be omitted by Google and thus recover your rankings. The details are at the site linked below. Note that it may easily take a month to see any results.
On this line, I'm not sure if I mentioned it yet, but there was a voice of wisdom from +Louis Simoneau on one of the episodes of the SitePoint Podcast ( http://sitepoint.com/podcast ). He said, that if Google simply ignored low quality links that they know of, spammers would still spam new links and some of them would not be caught as low quality links by Google. Therefor, spamming links would be viable. But now you have to select all the weak links yourself, so spamming isn't worth it anymore. Well said!
Here's the full article on the new tool:
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