I really like my minimalist slim card wallet. It only has three sections: one for cash, one for cards, and one for incidental paper. I use this latter section to hold receipts and a blank business card (adapted from the GTD darling, the Hipster mini). Tiny handwriting allows me to get two to three weeks out of each card (front and back), on which I write grocery lists, personal to-dos, driving directions, and contact information. New items are in black ink and old information is crossed out in red.
The iPhone presented an opportunity to make this card digital. It has a built-in Notes application, which certainly suffices, but it's also constantly connected to the Internet. Why not make a web-enabled notecard app, so I can access and edit this blank business card from any computer?
Pastecard is not an App Store app, but instead a small website built in PHP and Javascript. It's just a page with a text box that loosely resembles a blank business card. Click the box to edit it, and save your changes to look up later. I've now transitioned into using it primarily and only use the physical card as a backup. If you'd like to use it too, you can download the source and run an instance of your own.