This site looks good ,cheapness is not the best bet sometimes look at the extras and special offers. http://www.247regdomains.com
I do not recommend domainsarefree .com as your registrar. I registered a domain and without my knowledge, they started charging my credit card like $45 a month for a bs hosting account I had never even heard off. Also, the renewal fee is $15.00. I recommend highly recommend godaddy.com tough.
GoDaddy and 1and1.com are the largest and probably going to be on the cheaper end of domain names... GoDaddy tends to have a lot of specials going on too, like $1.99 .com names..
For me, Name.com is the one! FAST,CHEAP AND RELIABLE! Its not like godaddy which pops you with atleast 10 offers until you reach the payment page! *phew* I have never purchased a domain from GoDaddy for that reason
One more option is $1.99/year at yahoo. Here is the thread: http://forums.digitalpoint.com/showthread.php?t=1039140
This days I always purchase from name.com. I used to purchase from godaddy, but since the site loads very slow... with ads everywhere... I'd say nodaddy... Just my 2 cents