This 3×5 index card folio from Mead is something I picked up from Wal-Mart’s stationery department for under $3. The original cards that came with the folio were part of Mead’s “Collections by Cambridge” product. Online it goes for $3.45, but I found it for about $2. I didn’t like the flowery index cards it came with (not manly enough), so I swapped them out for a plainer-looking spiral bound set. Add another $1 for the replacement cards, and you’re still coming out pretty cheap. The pen is a Pilot G2 mini.
Having it with me is great for jotting quick ideas. It’s more robust and durable than a pack of Post-its, and besides, Post-its don’t come in a spiral bound format with a cover and room for a pen!