Namesilo. They're not resellers, but their price is pretty cheap and they provide API. From the other side, there's Enom.com and Resell.biz
Depends on what you plan on doing with the API..The best option would probably be Moniker, but getting an account with them that has API access isn't cheap. Aside from them, eNom is one of the better options - their cheapest reseller plan is a one-time fee of $195. Aside from both of those, you may want to checkout ResellerClub/DirectI who only charges $100 to active your reseller account. That money can then also get put towards registering domains (which enoms fee does not offer)..All three of these options have API's
Thanks for the comments, I also researched and found : opensrs.com and domainbox.com both offer the API as well, any one have experience with these 2 sites ?
Both of those should be decent, although I've never used either myself. I believe that Tucows owns OpenSRS and DomainBox i've read is decent before.
Recently I have gone with "Wild west domains" (Division of godaddy) It is the best i have seen, the price offered are low and API integration was easy too.. WWD offers it at for an yearly price. I used to be with "Reseller club" which was also good company, compared in prices for especially for domain WWD was little bit cheaper. How ever WWD also has a disadvantage. The privacy protection is at a price where Reseller club offers it free of cost. You can check the feed back on both and see which one is suitable.
You can try resellerclub.com. They offer API as well built-in supersite. Supersite allows you to show complete website and your users can purchase the domain directly from your website. - Kailash