Password quality algorithm

To improve the authentication process to access an aplication, I am looking for examples of password quality algorithms to use for password generation and changing. That is, each user could be assigned a minimum password quality value, such that a new password would have to meet or exceed that quality scale value to be accepted.
Lotus Notes has a process very much along this line. However, there are no details on how the quality algorithm is designed. Factors which would increase password quality might include password length, mixed case, numeric and special characters, etc. Any suggestions?