Who are thes mysterious 'aggregators'?

[ice1000]

I was reading this story about people who buy domain names.

Schwartz, for instance, directs the traffic from his 5,000 sites to one of the many small companies that serve as go-betweens with Google and Yahoo, the two giants that make this all possible. The middlemen, known as aggregators, do all the heavy lifting, designing the sites and tapping into one of the search engines’ advertising networks to add the best-paying links.

Who are these aggregator companies? I looked in Wikipedia for aggregators but came up empty.