We are a Web host using TotalNic for Domain registration. They allow us to use an interface with our name and some ability to set prices for purchasing Domains. We want a more sophisticated interface, which we would design, and better rates. We are looking for suggestions on who to contact.
And remember, when you're making your final choice, you'll be there a long time. If you register a lot of domains, the whole picture can change after awhile. For example registerfly... Right off the bat you have to pay $10, but 500 later, they're more in line with the cheapest of the cheap.
Also, be sure to look at the whole picture. DNS, their protection price (Some charge extra to lock domains!), the price of anonymous whois listings, some offer things no one else does, make sure you see all of those 'extras' and decide if they are important to you or not... Look at everything for the REAL best deal. And support isn't equal either, WHT's search is probably the best place to research a registrar's customer satisfaction.
eNom and Directi are my recommendations. For my own reasons I prefer one over the other. You will need to take a look at each and see which best fits your needs. Everybody gets hung up on price...so if that is your main concern the choice will be easy.