Betfair is the ultimate alround account for betting and gambling. It very simply gives the best odds on almost all events and also has regular free bet offers in both its sports book as well as its arcade section.
Currently if you sign up not only will you get a free sports bet of £20, but you will also get a free £10 arcade bonus with a turnover requirement  (WR) of just x1.

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It is also the essential online account you need if you are to start match betting. being an exchange you are actually betting against other punters and so wil almost always find better odds than the traditional bookmakers such as Ladbrokes and William Hill.