Pool, Snapnames, and Namejet are the big 3 but it depends on where the name was registered
Network Solutions names go to Namejet
Godaddy domains go to godaddy auctions.
Domains at Dotster, Register.com, Melbourne IT, DotRegistrar, DomainPeople, Moniker, DirectNIC go to Snapnames.
It doesn't hurt to backorder with all of them because you aren't paying unless they get the name