Godaddy and register.com are probably the two most widely recommended.
I'm using a tiny registrar
http://www.just15buckdomains.com/
They've been thoughtful, prompt and courteous. They're a lot larger now though, than when I began using them.
I find $15 to be rather high-50% more than I pay now. Since I have about nine domains, it adds up.
Register.com has a high price for renewals ($35)...avoid.
Network Solutions is well-known for abusing the domain search and registering domains once a customer has searched for it.
GoDaddy is cheap in every sense of the word. I have used them (for one domain), and found their interface atrocious, and their use of e-mail borderline spam. Also, most of the minor registrars you will find are actually Go-Daddy resellers (you can tell by the low price).
I researched domain registrars for about a month last year, and chose Namecheap.com.
I have found no reason to regret the decision. In fact, I searched high and low for a better registrar and there is none by my criteria (price, reliability, support and spam-free).
Price is under $9.00 using coupon codes (Google "Namecheap coupons" + monthyear) and includes free whois guard privacy.