KeePass is an open source password "vault". This allows you to use a single password (for the vault), yet still have distinct (preferably extremely hard to guess) passwords for each web site, online service, etc. (See why this is a good idea.)
KeePass is a Windows program, but the portable version will work under Mac OS X (and many flavors of Unix/Linux). Following these instructions, I downloaded Mono (open source .NET implementation)
The Mono installation notes for System.Windows.Forms (required by KeePass) say that X11 must be installed. (If you don't have X11, it's part of the Xcode Developer Tools. Sadly, it's no longer a free download, but if you have your OS X installation DVD/CDs you should find it there. It's also installed by default on 10.5, "Leopard".)
Install the Mono package, and be sure to save/print/keep the installation notes -- they include the deinstallation instructions, if you ever need to do that.
I unpacked the KeePass (portable) ZIP file into my home directory. For testing, I opened up a shell window, changed to my KeePass directory, and fired it up with:
It took a while (about a minute?) for Mono to start up, but eventually I had KeePass running on my Mac. I started going through the tutorial, but very quickly I found Mono and/or KeePass crashing or running away with 100% CPU. Boo! Time to do some debugging or look for another solution. :(