DNS Spider is a multithreaded bruteforcer of subdomains that leverages a wordlist and/or character permutation.
You can download it from the following link: https://packetstormsecurity.com/files/download/101410/dnsspider.py.txt
Source: https://packetstormsecurity.com/files/101410/DNS-Spider-Multithreaded-Bruteforcer-0.1.html