Flashlights and Headlamps for Adventure Racing
"Mandatory Gear" lights.
Some shorter races include a headlight/flashlight requirement. If you don't think you will be actually using the light, go for small and light.
Option 1: Micro flashlight. This might be a bit cute, but if a race director only specifies a flashlight, this should satisfy the requirement.
Option 2: A real (small) headlight like the Black Diamond Ion. At 1.1 ounces, including its battery, you can get around 90 minutes of decent light.
Real headlights
If you need a real headlight for a 24 hour or longer race, you'll want to do some shopping to find the best light for your specific race. There are many good lightweight options out there. Often a combination light that has a Xenon and LED combination is the best choice. The Xenon can be used when bright light is needed, and LED's will give enough light for many parts of a race, and result in almost no battery drain.
Full headlight reviews can be found here.