Tungsten is an ideal pinewood derby weight. It is a very dense (more dense than lead), hard, brittle metal. It’s most familiar use is in incandescent light bulbs.
The density of Tungsten is 0.697 lbs/in3.
If you are planning to use tungsten in your pinewood derby car, there are several ways you can buy it. The easiest way to use tungsten is buy pure tungsten welding electrodes. Do not buy “thoriated” electrodes, as they contain radioactive thorium. If you use 1/8 inch diameter x 7.0 rods, each rod will weigh a total of about 0.96 ounces. You can cut a 7 inch long electrode into 4 weights exactly 1.75 inches long — the exact width of a pw car. To cut, just notch them with a file, and easily break them off in a vise). You can get these at a welding supply center (like Nexair), or you can order them from Amazon: Weldcraft Tungsten Electrode Rare Earth 1/8
They usually come in packs of 10, and you will probably only need one or two sticks, so you might want to buy a pack, and split/sell them to other car builders (maybe you could get yours paid for).
Another option is tungsten powder. This is a little harder to work with, but you can put it in oddly shaped cavities, and you can add it to your car in very small increments. Good for getting the weight just right at the check-in. The 1/2 pound size is enough for at least 3 cars (check the approximate weight from the engineering drawings of your car to know how much you will need. You can get this from Amazon, too: Golfsmith Tungsten Powder 1/2 Pound
You can also buy tungsten putty. I can’t really recommend it, because I have never used it. If you have used it, please comment below and let me know what your experience has been. Pinewood Derby Tungsten Putty
This is new: Tungsten Incremental Weights, 3 oz Plate