Yes, bookmakers are taxed irrespective of win or loss. The two biggest books by far are Soccer and Horse Racing. The next books down are probably Rugby Union, Golf and Boxing. You can pretty much bet on anything though. Largest book makers are still dictated by the highstreet (Ladbrokes, Coral, William Hill and Paddy Power), however there are a few big online bookmakers that are hugely popular (Bet365 and Bet Victor are two such examples).
RE: Is Online Sports Betting/Gambling Legal in the United States? Dispelling a Common Myth