Exploit Prevention Labs recently found that a few of Google's sponsored links -- for organizations like the Better Business Bureau -- were redirecting to a Russian Web site attempting to load post-logging software via an exploit Microsoft patched last year. The offending links have since been removed by Google, but EPL's folks have posted a video over on YouTube of the exploit in action.
Also, the LinkScanner application referred to in the video comes in both paid and unpaid versions. You can check out the difference between the versions here. I'm trying out the free version now and will report back in a few days.