I've considered this before. It's a nice idea, but it suffers due to the arbitrary length and complexity requirements that many sites place on passwords.
Use lowercase hexadecimal as the "baseline" password. From there, truncate to the length requirement, and add symbols to the end to guarantee complexity requirement.
For example, "masterpass gmail.com" will md5sum to "194b52e5". If a password requires symbols, add a "!" to the end. If it requires a capitol letter, add "A" to the end. Add in the order of "number->letter->symbol". So... a site that requires numbers, capitol letters, and symbols would be:
"194b52e51A!". (The hashed password, followed by '1A!')
That does't sound simple, because I'm not going to remember the site specific password requirements for each of the 250 logins I have within my password manager.
