Traffic measurument with TrafficEstimate.com

TrafficEstimate is a free service, and they provide just one type of metrics. Also, their numbers used to be highly correlated with Alexa, but this is no longer the case.

Now they provide the most accurate numbers, compared with Compete, Quantcast and Alexa. They provide the most accurate traffic summary snapshot for all our Internet properties and also about our competitors or publishers that we might consider for advertising purposes.

Besides TrafficEstimate, Compete, Alexa and Quantcast, is there other similar companues measuring traffic statistics? Which ones do a good job identifying real users from robots and other artificial traffic (e.g. clear gif sessions from external domains).