Instead of keeping their own book, TradeSports sets up futures style contracts with cash settlement that pay off based on the outcome of a future event. (I'm reminded of Idea Futures, except for the most part they're doing pretty ordinary sports bets.)
In addition to sports, they're running a number of contracts on future economic statistics, elections and other such things.
(One hopes that perhaps someday they'll do Idea Futures. There are places like the Foresight Exchange that do idea futures, but so far as I know none use real money.)
One reason I mention them is because they're running a set of contracts on the current U.S. presidential election, much like the Iowa Electronic Markets, except they're not a small scale academic experiment.
Currently, it appears that the people on TradeSports collectively believe George Bush's reelection is a 50/50 shot, which is pretty much the same prediction that the IEM and the Foresight Exchange.