Leo Laporte gave this to me yesterday while doing an ad for TWiT: Watch the show and ad here. It's called a Trackr. Each one is about the size of a quarter and costs about $30. (Pro tip: Buy it here and use the promo code TWiT for a better price.)
After downloading the free app, you press a small button on the Trackr to pair it with your phone, and it saves you from losing or forgetting whatever you attach it to (keys, wallet, spouse, etc.) -- and also from losing or forgetting your phone. If you leave without your phone, the Trackr alarm sounds.
If you leave with your phone but without whatever you attached it to, your phone alarm sounds. It gets better: You can map your house, and use a web browser to find your stuff in your house. And if you truly lose something, you'll be alerted and shown on a map where your stuff is each time another Trackr user gets near it.
I attached it to the underside of my backpack (which contains everything -- camera, laptop, etc.), so now if I leave it behind or leave my phone behind, I'll be alerted. Nice! Watch Leo's ad for more information.