We've shared a few ways to add your contact info to your gadgets in case you lose them, but Apple weblog TUAW shares another clever method: using Onyx, you can add that info right to the login window.
One of Onyx's many nifty features is adding a few lines of text to the OS X login screen. TUAW suggests using this to add your name, address and/or phone number. That way, if anyone finds your misplaced laptop (and they're kind-hearted folk), they know exactly who to return it to after powering on the device. This is especially handy if you require a password to log in to your account — if they don't know it, that login screen is the only screen they'll see!