Namehceap.com has been my favorite domain registrar ever since one JD Swanson recommended it in his ebooks (by the way, I am not a JD fan in any way).
I am someone who firmly believes that one shouldn't keep all the eggs in one basket (the reason why I have two autoresponder accounts and two hosting accounts with different service providers), but I am yet to find a domain registrar similar to Namecheap. :D
If you want to registrar new domains in September 2008, here is the coupon code you should use : BACK2SCHOOL. With this code, you will be able to registrar .com domains for as little as $8.41!