Compete.com provides "quantitative analysis of web traffic".
Compete monitors web traffic via user toolbars much like Alexa does. They extrapolate the data they collect from the users who've installed their toolbar.
Compete provides information on traffic history, how sites compare against the competition, trust scores, ranked lists etc. More on the Compete.com FAQ page.
Compete is very biased towards the US as that's their core monitoring market. It suffers from other significant defects. For years Compete has been ignorant of the fact that Limited companies in the UK use .ltd.uk domains as in companyname.ltd.uk. It treats the ccTLD of .ltd.uk as a domain and all these company sites as sub-domains of ltd.uk. (It doesn't display the same ignorance for .co.uk.) So for years it's been combining the stats for thousands of websites and reporting them as one!
Why bother with Compete? As with any tool, it's about how you use it. On its own, it may not be a conclusive indicator of anything. Taken together with a lot of other data for a variety of sources it can help piece together a better picture of a site's performance.
Compete on its own plays a relatively insignificant part in the assessing of a site for purchase but in our valuation reports we provide the graph and leave it to readers to pursue further research if they wish.
You can run compete searches here.