Debug Card
The debug card is a creative-only item that was originally only intended to make debugging things easier, by automating some processes. It has since gotten a bunch more functionality, making it quite useful for custom map-making.
Note that you can use sneak-activate while holding the card to bind it to you or unbind it, meaning runCommand will be performed using your permission levels instead of the default OpenComputers ones.
A debug card can receive messages similar to a linked card, firing a debug_message event. You can send such a message using either another debug card's sendDebugMessage or the Minecraft command /oc_sendDebugMessage (or /oc_sdbg).