If you want to break a hash, such as a password hash. Then you need a very large lookup table. John the Ripper is commonly used to break passwords using a dictionary, this is a very good method espeically if its a salted password hash.
Another approch is using a Rainbow Table, however these take long time to generate. There are free rainbow tables accessible online.
Here is a python script to perform an md5() brute force attack.
