Another vote here for the passport card. It's usefulness is not limited to transiting TSA checkpoints; I am not a frequent traveler as most FTers are, but I have found my passport card very useful in a number of other situations where I need a second form of photo ID - it's issued by the federal government, and is RealID compliant, so it's acceptable to all sorts of government agencies and private companies. And as several others have mentioned above, it doesn't have your home address on it, which is a plus to me. It's also wallet-size, and laminated as tough as a DL, so it'll take a beating and won't dissolve if it gets wet.