My iMac is my holy shrine and no one except for me is allowed to touch or even to use it. That makes it easy to store all passwords in Safari.
Currently, many sites are integrating multiple Sign-In technologies with accounts from other sites like OpenId, Facebook, Twitter, Flickr, Yahoo and more. That can be a cool thing, but require to re-authentificate by logging in to your primary account.
One problem is coming with that! The site you need to put in your username and password is not the same as you would visit to login, where the formdata of your browser is stored. And how do you get your password back?
Normally, you could request a password reset, but that takes time, because you need to wait for the email, confirm the link and sometimes to change the password. That is too nasty and it is much easier. Use the Mac’s Keychain Access utility.

Keychain access
1. Go to Application/Utilities/Keychain Access.
2. Put in the domain of the website your forgot the password.
3. Doubleclick the appropriate name.
4. Put in the checkbox next to “show password”.
5. If you are asked for your Mac password, put it in.
6. See the password and use copy & paste.
Leave me a comment if it helped you.