Google just announced its ad exchange:
The DoubleClick Ad Exchange is a real-time marketplace to buy and sell display advertising space. By establishing an open marketplace where prices are set in a real-time auction, the Ad Exchange enables display ads and ad space to be allocated much more efficiently and easily across the web. It’s just like a stock exchange, which enables stocks to be traded in an open way.
There are some existing competitors, but Google’s entry may be game-changing. Lots of research problems beg themselves.
Turing's Invisible Hand 
Another competitor, but aimed at small webcomic sites instead of large sites: http://www.projectwonderful.com/
Does this mean that prediction markets for words like this one http://pretweeting.com/ could now be valuable? If someone can predict in advance what words will get popular could they make money?
Hmm, I’m sure that as usual here in the UK we won’t hear of this for months to come.