www.1and1.com — The least expensive (yet still reliable) domain registrar I've found, at $5.99 per domain per year.  However, they have VERY slow customer service response: in my experience, they rarely respond within 24 hours, and it often takes more than 48 hours; occasionally, they've never responded to my question.  That may make their low price less appealing for some.

www.powerpipe.com  — They offer decent support and service, usually within 12-24 hours.  Yet they still offer a great price at $7.99 per domain per year.  Their biggest drawback is that they don't automatically renew your domain name — so if you forget to renew it manually, you could potentially end up losing it, forever.  On the other hand, they do send out frequent renewal reminders to the email address listed on the domain account.