Hey ive been playing around with
Betfair.com: Online Betting, Sports Betting, Horse Racing, Football | Bet Poker, Casino & Games recently and they seem to be quite good with their odds, you wager against other members so they take less % of the winnings than tote etc.
If you decide to sign up use my referal code which is: HFCPHXDHF
That will give you $50 free once you have made $50 worth of bets, and the bet doesnt have to be on one race like other places although there is a minimum of $6 per bet.