FYI, I just launched bitbook.ie (
https://bitbook.ie). It's vig-free, parimutuel sports betting. It has scores and informational Vegas odds (the real odds are derived from user bets) for the World Cup, NBA, NFL, MLB, NCAA Football and more. The Blockchain is the Book.
No accounts or deposits. Bet by sending a transaction to the specified Bitcoin address. All bets and payouts are shown in the Ledger for verification by the public.
It's as decentralized as I can make it, but I would very much like to decentralize it further using coloured coins / smart contracts. Let me know if you can think of a clean way to do that.