KeePass is an Open-Source password safe utility for securely storing passwords. The interface is simple to use and works similarly to PasswordAgent, but it’s free. I’ve worked with the Windows version for a while and haven’t had any problems (yet). There is a project for a Linux/OS X version but it’s not yet available in a native OS X binary. Currently, you must compile the program to get it to work in OS X. I’m going to attempt to compile it and give it a run on my Mac.
KeePass – http://keepass.sourceforge.net/
Update: As of October 2010 I have been using KeePass for several years with success in Windows and OS X. There is now an OS X specific binary available.
Updated 08/24/2011: KeePassX is still my preferred secure password database application.