Quantcast is an interesting new approach to the Alexa problem. Like Alexa/Comscore/etc., they have a panel of users they track and then extrapolate rankings from. But they have an alternative to correct that: Put their tracking code on your site.
If they get a critical mass of sites to do this (and they can keep up under the load, and they're not evil, etc.), it could be pretty interesting.
They also have a richer set of data than Alexa, with demographic info (apparently from their user panel).
I'm going to throw the code in Odeo and see what happens. (Currently it's ranked 15,500will be interesting to see which way it goes.)
Update: In fact, after putting the code on odeo.com, its rank jumped from 15,500 to (currently) 2,900, which seems like a huge jump. Possibly because we have a lot of non-US traffic? Possibly because their non-quantified rankings are way off?