Godaddy is cheap and reliable.
You can get one for free from Microsoft if you sign up for http://smallbusiness.officelive.com/, although you may not be able to move it elsewhere later, they say:
If you signed up on or after February 11, 2008, your domain name is free for the first year, then you will be charged $14.95 annually at renewal thereafter. Microsoft will automatically renew your domain name for you each year unless the domain name is canceled prior to the renewal date.
Google also sells domains with google apps:
One year registration costs $10 and includes these benefits at no additional cost:
Google Apps Standard Edition pre-configured for your domain, including Google Sites and Gmail for custom email addresses.
Full DNS control and management
Private registration to protect your personal information
Domain locking to protect you from unauthorized domain transfers