Here's an Open Source application this is even more multi-platform than usual.

KeePass:: The Open Source Password Safe

The original version was written for for use with Microsoft Windows. However, its author lists unofficial ports for Linux/Mac OS X (X11 required for the Mac), Palm OS, Pocket PC (Windows Mobile), and even the USB flash memory U3 portable smart technology format.

The application's official description reads: KeePass is a free/open-source password manager or safe which helps you to manage your passwords in a secure way. You can put all your passwords in one database, which is locked with one master key or a key-disk. So you only have to remember one single master password or insert the key-disk to unlock the whole database. The databases are encrypted using the best and most secure encryption algorithms currently known (AES and Twofish).