Another one of those things you need. Might seem small and expensive, but it isn't a thing you're supposed to use up. One of them should last 'forever', but it doesn't hurt to have a couple stashed away just in case. If you need it - get it. If you don't need it, get one anyway.
Full name: 3/8" to 1/4" Tripod Thread Adapter Screw - Silver
Webshop: DX.com (Deal Extreme)
Price: $1.40 (Free shipping)
Nice little 3/8" to 1/4" converter / adapter for use with ball heads and all kinds of other photographic equipment.
Feels a bit too short for some uses. Small and easy to lose, but buy a couple of them and put them on a string or something so you don't lose them. On the other hand, when in use it should be almost impossible to lose them.
If you need it - get it. If you don't need it, get one anyway.
Buy it here