Horsemart, Horsequest, Horse Hunter, Horse Exchange, Horsedeals more or less in that order. Again, don't look at H & H much because you can't narrow your search down enough. And pricewise, if you were looking for a horse say 0-1000, you get hundreds of horses coming up with POA, which I never even bother to look at!
Scot Horse I use a lot - it has lots of Scottish horses on it. Sometimes Horse Deals. I find more national ones barely have any in Scotland for some reason! I dislike HorseQuest - just the website design bugs the hell out of me so I never use it!