Compete, Inc. is a B2B firm that conducts consumer-oriented research for large clients. Now the company has released SnapShot, a free tool that anyone can use. It operates like Alexa or Google Trends to display relative traffic, as well as several other metrics such as page views and time on site. Here's an example.
According to Compete, there's a qualitative and quantitative difference between this tool and Google Trends or Alexa, because it leverages their entire 2 million person panel. See Compete's Alexa comparison.
There are a few limitations: it's U.S. audience only, sites with fewer than 10,000 monthly uniques are excluded and you can only compare three sites side by side. But out of the gate it's a much better and more accurate tool than Alexa.