I imagine there are lights designed for the shape of helmet that I have, but I'm far too cheap to get one. I have one on my handlebars, though.

After riding on a trail at night with my brother, I realized that having a powerful helmet light would be really handy, because he was having a lot less trouble finding his way around, due to having a bit more than moonlight.

The light on my handlebars serves little more purpose than alerting someone ahead of me that I am there, it's not very bright at all.