there's 3 other major ones, betfair is by far the largest. the alternatives have some pro's and cons though.
betdaq: 2nd biggest one after betfair, they have the same standard 5% commission and are primarilly used for horse racing, also football and tennis. they are pretty much useless for mma however. also like betfair you can reduce the commission you pay the more you bet.
matchbook: have decent liquidity on major sports, not as much as betdaq but more spread out among other sports. US sports markets are more popular on here. they have decent odds for mma at times, but the liquidity is lacking often for big bets. commission is 1.5% on bets you take, 0.75% on bet's you offer that get matched (last time i checked anyway)
smarkets: Smarkets is relatively new compared to the other ones 2% base commission, good liquidity on football, tennis,horses. but a limited amounts of smaller markets compared to the others, and most of them aren't worth looking on unless your trying to trade them. It's extremely popular among ppl doing matched betting. again utterly useless for mma, although they did have the rousey/nunes market up yesterday. but I imagine the amounts matched are ridiculously low.
betting on the exchanges for mma is still relatively new (in terms of liquidity available) but over the last year or 2 there have been big improvements, I'm hoping in a couple years it will grow to a point where ppl who have been affected by bookmakers gubbing your accounts (I'm banned on at least 15 books so far, probably more if I count smaller ones) will be able to bet almost exclusively on the exchanges. I know a few ppl here are having similar problems with books.