Check out this product from Hobbyking.com. It is a Turnigy Battery Charge Marker. It has an adhesive back, which sticks to your battery. After you charge you battery, you just slide the indicator to green. When the battery has been discharged, you move the slider to red. This is a neat idea, that will help you keep track of your batteries. They come 10 per pack.
For more neat products, check out Hobbyking.com