Cranks up the number of simultaneous requests, but nic.io just barfs at you after a short while.

check_free_io.py

# works on my machine... OS X 10.8.2, Python v2.7.3

import sys
import subprocess
import multiprocessing

def checkName(q):

    proc_name = multiprocessing.current_process().name

    print 'Starting process', proc_name

    while True:
        domain = q.get()

        try:
            out = subprocess.check_output(['whois', domain])
            if 'Not' not in out:
                print domain

        except Exception as e:
            print "Other Exception", e

        except KeyboardInterrupt:
            return


if __name__ == '__main__':

    print 'Starting'

    wordlist = 'words.txt'

    if len(sys.argv) > 1:
        wordlist = sys.argv[1]

    procs = 10

    queue = multiprocessing.Queue(procs)

    p = multiprocessing.Pool(procs, initializer=checkName, initargs=(queue,))

    try:
        for line in open(wordlist, 'r'):
            domain = line.strip() + '.io'
            queue.put(domain)

    except KeyboardInterrupt:
        sys.exit()

    finally:
        p.terminate()