Chris Abraham gave us a great little writeup on his blog as a prediction market he recommends:

Really basic but also really easy to use. Instead of being a very specialized and highly-categorized prediction market based on politics or entertainment, this market is free form, which I think is a much more powerful strategy. The look and feel are very basic but I was up and running within minutes. Also, they offer you lots of money to trade at first and I wasted it all in one trade called, "Lindsay Lohan Sex Video," which is a market as to whether "a sex video of Lindsay Lohan surfaces on the Internet." I really like it.

Check out the rest of his blog, he covers a wide range of topics including social networking, politics, and blogging. Good stuff!