Betfair

Betfair is the world's leading betting exchange, a concept it pioneered.

All bets on Betfair are placed there by users who either want to bet for a selection or outcome in the normal way (back), or bet against a selection or outcome (lay). Bets are matched between people with opposing views.

When you "back" a selection (be it an individual, a team, horse, dog or other), you are betting that it will win. This is just like betting with a conventional bookie.
When you "lay" it, you are betting against it winning. For example, if you're betting in a market on which team is going to win the Premiership and you lay Man Utd, you offer odds to other punters who wish to back Man Utd. If Man Utd don't win, then you pick up the backer's stake. If Man Utd wins, then you pay out. This is what bookmakers traditionally do.

Although Betfair knows who is on each side of every bet matched through the exchange, you never know who you are betting against. Your privacy and the confidentiality of your bets is maintained by Betfair's secure site.

If you want better odds than are currently available, you can place an order for a better price, but be realistic because there has to be somebody prepared to lay the bet at those odds, and vice versa.

Take a look at the odds before an event and you will see them change as Betfair members back and lay their bets. The odds often improve nearer to the off as the number of people in the market increases.

http://www.betfair.com/