Reseller club and eNom are two I have heard of. I am not sure how good reseller club is but eNom is very popular.
Do you know of any free or cheap domain reseller programs?
I use Reseller Club, The only complaint I have is the interface is very counter-productive. If you have WHMCS or ClientExec, it comes with a free Domain Reseller Account. Or you can contact me if you need a direct link to Reseller Club
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I was involved in a hosting company once and I think we were with enom. They were pretty good. If you want more details let me know.