Add sound to anything
I feel a little uneasy writing this one up, since it's such a simple idea, but with Halloween coming up I feel it's my public duty to remind people that you can add sound to your costume or decorations for less than $50.
Now that the iPod's heyday is over, you can pick up a halfway decent MP3 player for as little as $20. Slap it together with one of those ultracompact portable speakers, and you have an inexpensive little sonic grenade that can bring the miracle of music to just about anything.
I know I'll get some comments here calling me Captain Obvious, but hopefully I can help at least one person make his or her Halloween costume extra awesome, or give a new parent some sleep by making an improvised white noise machine.
Also, before you say it--yes, I know they sell speakers with built-in SD card readers for MP3 playback. The problem is that they're typically not as cheap as buying an MP3 player and a speaker separately. If you're a gadget pack rat like me, you may even have the ingredients already. You're just a Velcro strip away from adding a soundtrack to your bike, costume, yoga mat, Barbie Dream House, coffeemaker, Roomba, or whatever else you can think of.