Niterider's small lights have always been favourites. Even this low cost 'commuter' light has good features, and enough punch for playing rough if you get a helmet mount.
Despite the claimed output being just 220 lumens, the Micro puts out enough light to keep more expensive and powerful lights in sight. The beam is very focused, so you need a helmet mount (it uses the same £12 one as the rest of the Lumina range) to tackle technical twisty trails.
There's a run time indicator built into the soft switch to help manage longevity, but battery life is limited. You'll have to save full beam for the sketchiest bits of trail if you're on a decent (two- to three-hour) ride.
Niterider Lumina Micro 220 beam shot