I used http://www.resellerclub.com http://www.resellerclub.com. $9.99 for .com, $3-$5 for .biz, .org, .net. Their API is pretty straightforward t to use, and

I used http://www.resellerclub.com http://www.resellerclub.com. $9.99 for .com, $3-$5 for .biz, .org, .net. Their API is pretty straightforward t to use, and supports availability checking, registration, and DNS settings. I used it for an experiment in automating domain registrations, article writing, and site creation based on Google keyword prices.