Why would you keep my business card?

[Weston Meckes]

What could I have printed on the back of my business cards (I'm a realtor) that would make you so inclined to hang onto it? Something to provide value, preferably. A resource. What's something that you run into in the field that most people need to look up, and it could be useful to be able to simply pull my card out and check the back? I appreciate any input or suggestions!