DNS Spider is a multi-threaded bruteforcer of subdomains that leverages a wordlist and/or character permutation.
You can download it from the following link: https://packetstormsecurity.com/files/download/148377/dnsspider-1.0.py.txt
Source: https://packetstormsecurity.com/files/148377/DNS-Spider-Multithreaded-Bruteforcer-1.0.html