From the wikipedia article (http://en.wikipedia.org/wiki/Salt_%28cryptography%29):
“Salts also help protect against rainbow tables as they, in effect, extend the length and potentially the complexity of the password. If the rainbow tables do not have passwords matching the length (e.g. 8 bytes password, and 2 bytes salt, is effectively a 10 byte password) and complexity (non-alphanumeric salt increases the complexity of strictly alphanumeric passwords) of the salted password, then the password will not be found. If found, one will have to remove the salt from the password before it can be used.”
In the context of extending the length of the stored hash and preventing the obviousness of two identical passwords using a unique hash for each password can be done simply, as previously suggested, by appending the hash of the username or other such related constant.