I use register.com for my own registrations, while most of my clients use GoDaddy.com.

Register.com charges full price, $35, but I think they provide better service and are more reliable.

GoDaddy.com charges a cheap $8.95. They're OK, but for such a low price you can't expect much attention. They're a favorite among people who buy lots of domain names and need the low price, such as people who operate those cheezy fake search pages you get when you mistype a domain name. There are actually people who own 100,000 domain names, so you can imagine how it matters to save $26/each to renew the registrations.

Both have the standard features you should look for in a registrar. Most important is self-service DNS, which means that you can maintain your own records in the Domain Name System and don't have to pay/cajole your ISP to do it. Self-service Web hosting and site creation tools are important unless you plan on paying a Web designer to bulid the site. Ditto an email service to go with your new name. The DNS bit is included, but site creation and email will be extra-cost items.