Recs.
Updated
The Cygolite Expilion 850 is a great headlight for those extra dark or extra long trips. It emits 850 lumens of light on the highest setting. It has a hard mount that can be attached to the handlebar, and comes with an additional mount for your helmet.
SpecsUpdate
Pros
Pro Excellent for long rides
For commutes longer than 30 minutes you can get more battery life out of it by running it on a lower setting- the Expilion will run for 3 hours on the 425 lumens setting.
If you like to always have a full battery, the Expilion has a swappable 3.7-volt lithium-ion battery, though you'll have to purchase the spare separately.
Pro Great for darker rides
The Cygolite Expilion 850 is one of the brightest light for the price. The highest setting emits 850 lumens of light which is great for roads with no lighting at all like in rural areas or off-road trails. Even on the lower setting it's still bright enough for non-illuminated bike paths.
Pro Has a handy mount that won't require you to re-aim the light each time
The mount on the Expilion 850 clips around the handlebar, and then you slide on the light itself on the stationary mount. That means, you only have to aim the light in the direction of your choice once, and it will maintain it's direction when you come back to it.