Bet365 is propably the No1 destination for sports betting on the internet. Nicely designed, with huge selection of different sports and events do bet on, and lots of contests and extras. Available in US Dollars, Euro and UK Pounds.
Sign-up bonus: 15% Initial Deposit Bonus up to $200 free
Extras: Login with username: guest and password: guest to see exactly how their betting system works.

BetShop is available in 6 Languages and you can bet in almost any sport event on earth. From horses to F1, American Football, Baseball, Hockey etc. Check it out, they are offering £30 Free to try their services without deposit required.
Sign-up bonus: 10% Bonus on new accounts
Extras: Betshop is offering a "virtual" £30 UK Pounds in a FREE betting account to anyone who wishes to experience the joy of betting! And if you reach £500 with these virtual money you will win a real betting account.

Sports Betting is the simplest and most popular destination for sports betting on the Internet. 10% for refferals, 5% bonus for Amarican Football bets, 10c baseball lines and more.
Sign-up bonus: 10% Free for initial deposits
Extras: Chance to win $100,000 every weekend, Free Contests everyday, 24/7 phone and online customer support.