Windows/Mac/Linux: Firefox extension and Internet Explorer add-on LastPass is a secure password manager for all your web passwords. Like other web-focused password managers, LastPass puts all of your individual passwords behind one master password. When you type in that master password, LastPass can then automatically log you in to any web site you visit with saved login credentials. Even better, LastPass syncs passwords over the internet, so all your saved passwords on your work computer, for example, will always be synced up on your home computer. I haven't been using it for long, but so far LastPass works seamlessly with Firefox and exactly as expected.
If you're creating a login for a site for the first time, LastPass can autogenerate a strong, secure password for you (remember, all you have to remember is your LastPass login). LastPass isn't the first password manager we've seen by any means. Previously mentioned applications like KeePass (or the Mac version, KeePassX), PassPack, 1Password, and Sxipper all aim to manage and secure your logins and passwords. But with its seamless syncing and cross-platform browser support, LastPass could draw a big audience pretty quickly. If you like the look but have already spent a lot of time with another password manager, LastPass supports importing passwords from KeePass, Sxipper, Roboform, and others.
Keep in mind, as always, that in order for syncing to take place, your passwords are stored online—meaning that you're required to place a certain amount of trust in the hands of the folks at LastPass. If that's something you can live with, it looks like a strong choice. If not, you may want to stick with an open source local option like KeePass. LastPass is free, works with Firefox and Internet Explorer.