A lot of my clients use Yojimbo to securely store their credit card numbers, passwords, and other information. I use it, too, but find it to be a somewhat drab application. On the other hand, Delicious Library is attractive and fun to use, but it isn't really something you'd want to use to store private info. If these two apps mated and had a love child, it would probably look and act a lot like RiftVault.
RiftVault is a new Leopard-only secure vault application from EdgeRift that is currently available in a pre-release version. It looks mahvelous and features 256-bit AES encryption to keep your private stuff from prying eyes. You can store credit card info, frequent flyer account data, passwords, insurance information, or short notes that you don't want to make public. Any document can be dragged into the Safe Deposit Box to be compressed and encrypted. Up to 2 GB of items can be stored in the Safe Deposit Box.
Sound interesting? Download RiftVault for free during the pre-release period, and/or pre-order the release version for $33 -- 33% off of the $49 retail price.