While SIM cards hold all the stuff address books used to, some people just can't part from that old ink-and-paper. The SIMbook lets people know that you're aware of modern technology, yet you choose to go retro.
Pretty simply, it's a small pocket notebook in the shape of a SIM card, both in unformatted (notebook) and formatted (address book) styles. Each SIMbook costs about $6. [Address Book, Notebook via Nerd Approved]