Know of a few
YOUBET.COM, EXPRESSBET.COM, and WINTICKET.COM are all OTB's so to speak, they comingle their wagers with the on track pools. You do get full pay outs, but you also recieve tax statements (W-2G) on qualifying wins, plus they do withhold taxes on major wins of 5000 or more. They are not "books."
The books usually put a cap on winnings, like with WSEX.com, they will only pay out say 300-1 on a tri, or $600 on a winning 2$ tri, even if it pays well over that.