GoDaddy is perhaps the most popular. It's a great price, and they're very flexible. I've registered 2 or 3 domains with them now, and I've had a first-class experience every time.
As they've grown, I've noticed that they try and "sell" more things to you that you probably don't need, but that's to be expected with any growing company. I would recommend them, especially for your first website.
