betehorse is the best out there i think
they pay full track odds -- they have all races on, so you can bet right like you were at the track
other books mentioned in this thread add time to your ticket, which means you can't bet on post -- which is when i'm looking to put in my plays -- NOT TO PAST POST --
also, betehorse gives up to 5% back rebate on your horse action each week, depending on volume -- any serious horse player has to try them -- and they don't feed into the pools at the tracks ---
downside of ehorse is limits -- not the highest limits, but still sufficient for 98% of the horseplayers out there