If you can’t remember a password for a website (or anything else for that matter), all your passwords are saved in the Keychain Access utility (which probably isn’t news to you), but the cool thing is you can do a Spotlight search from right within Keychain to quickly find the password you’re looking for.
Start by looking inside your Applications folder for the Utilities folder, and inside of that double-click on Keychain Access. When it opens you’ll see a search field in the upper-right corner. Type the name of the site you’re looking for, and it will appear. Double-click on the result and an info dialog will appear, and to see your password, turn on the show Password checkbox.