Surprised no one has chimed in with eNom yet. eNom is without a doubt the cheapest and best, as you can procure a $8.14 reseller or retail account there rather easily. Transfers in, out, and within are fast and easy. Most domains you buy here from other resellers will have been bought at eNom, so it's easier to keep them all under one roof if eNom is your choice of registrar. Funding is available by PayPal, major credit cards, checks, etc.
If you need an account, PM me your WHOIS info and a selected username, and I will set one up for you with $8.14 purchase rates for major extensions.