I started using GoDaddy few days ago, after I found out that .Info are just $1.99 , considering I used to give $11 for them few months ago... It's a great deal.
NetFirm and Yahoo both are cheap but I would keep myself away from them! You can go with GoDaddy / NameCheap / Registerfly If you are planning to register BIG amount of domains then you should become reseller of enom / resellerclub
ipowerweb.com hostgator.com acenet.net Go Daddy I have domain names with all of them now hostgator has an awesome package called baby croc you can use most scripts with that 200 gigs of harddrive space unlimted domains - etc all are faily cheap without coupons