Compete.com just released a Web Developer API that went live this afternoon at http://developer.compete.com/ .  In fact, Compete's API tool has already been used for Domaintools.com which provides Traffic Rank numbers and a link to a Compete Snapshot of the domain . 

I will say that, piece by piece, Compete.com and Quantcast.com (along with Alexa's recent update last weekend) are chipping away at HitWise - since everything that you can get from Compete, including the API and therefore, interfacing with the data - is FREE - and as the data gets more granular ….. the metrics will get more specific, more accurate for that domain.

So, right now you can create code via the Compete.com API to figure out if any domain is trusted and the traffic count/ranking. 

Today, it looks like what you can get via the API is not much different than what you can get by entering a URL in Compete.com and querying for the values of traffic, time spent on page, etc.  I think in the future - more can be added.

I can see the API useful for the current crop of SEO/SEM tools like WebCEO, OptiLink/OptiSpider, SeoElite 2.2 and many free online SEO/SEM tools that you can find all over the place like We Build Pages, SeoToolSet, SeoCompany, etc. But you never know - and more applications get API access to Compete's data it's possible that some of the smaller search engines might also use the API.

There are some Branding Usage guidelines - which should be no big deal to comply with.  If I were Compete, I'd make everyone that uses their API say "Powered by Compete" and put a icon lick (with some text also) back to Compete.com - which will be great for their search rankings and traffic.