Yahoo Domains has a great offer going on right now. You can register .com domains for the VERY LOW rate of $2.99 per domain. Be advised that Yahoo Domains calls itself a reseller, not a registrar. Because of this, you can't "push" the domains to another Yahoo Domains user like you can at other registrars.
Also, be very careful when purchasing domains at Yahoo: You have to go into the Domain Management panel on their website and cancel your services, even if you transfer your Yahoo Domain somewhere else. They are sticklers about this, so consider this fair warning if you register any names at Yahoo Domains.
I also suggest you plan on keeping your Yahoo Domains at Yahoo for most of the first year, and then if you want to transfer them away, do it a month or two before they expire, so you can take advantage of the free year's registration given by many registrars when you transfer.