This is an old favorite of mine.
So, let's say that you've got a remote control - and I'm talking specifically about remote controls that utilize infrared (IR) technology, not raido frequencies (RF) - and you need to see if the batteries are working or not. Normally, you'd just point the remote at whatever it is that it controls, and test it out, right? If it works, the batteries are good, if not, they need to be replaced.
But what if you don't have access to the thing that it controls? You've only got the remote. And you need to make sure it works. Sure, you could just put fresh batteries in just to be sure, but where's the fun in that?
Here's what I do when I'm in that situation:
If I'm at my computer, I fire up PhotoBooth or iChat, or any other app that will turn on the built-in video camera and bring the picture onscreen. If I'm out and about, I'll just use my iPhone's camera app.
Next, I simply aim the remote control right at the camera and press a button or two.
If it's working, I'll see a white-ish/purple-ish light blinking from behind the IR cover on the remote. If it's not, I won't see anything.
Pretty neat, eh?
This has come in quite handy a number of times, especially when troublshooting.
You can also use this trick to see other IR lights, like the ones used on small closed-circuit security cameras.