Really interesting site I just found out about from the regularly excellent Data Mining blog.
This site has the potential to give us some numbers on the 'blogosphere.'
At the moment, the only effective measurement on blogs is through Technorati and Nielsen's Buzzmetrics. Spotplex is offering the ability to measure actual readers (or at least page impressions).
It'll be interesting to see how well this service does. It has the potential to compete with both digg and google analytics. While it is doing this, it will enable us good ol' advertisers to work out how many people are reading these blog things, and give us some fantastic graphs to show what people are actually looking at.
Can't wait till this gets integrated with Swivel!