Safari 5 (Windows/Mac): It's still early in Safari's extension era, but a very nice addition to the ecosystem has just landed. LastPass, the any browser, any OS password system, has an early extension out for Apple's browser.
LastPass appears to work on Safari much like it does Firefox or Chrome—you get an asterisk-like button added to your toolbar, and after logging in with your master password, LastPass handles all your logins and form filling from then on out. You can pull up your LastPass vault for storing and retrieving other sensitive keys and codes as "secure notes", and, heck, that's about it. It's a more secure system then storing your passwords in Safari itself, and those using multiple computers throughout their day or week should check it out.
The LastPass extension is a free download that runs on Safari 5 on Windows or Mac. You'll need to enable extensions in Safari to use it, but once you do, it's a pretty simple two-click installation.