This little multithreaded tool, written in Haskell, searches quickly for free domains. Just give it a list of words and top level domains. It uses DNS lookup, so it is not 100% reliable but it is very fast. Below find a short example of FreeDom scanning for free domains in 100 threads simultaneously.
You can use whois, which is unfortunately painfully slow and often throttled, to cross check the final results you are interested in.
You can find the source code in my Haskell git repository: