If I were forced to pick a favorite component or accessory for my bicycle, it would easily be my Schmidt dynamo hub with its accompanying headlight. (a bit of a review following the pics)
Drive-side view of my Schmidt dynohub:

I happen to have the Schmidt made E6 headlight, which uses a halogen bulb, as shown attached to my Nitto front rack on Cross-Check frame:

Here's the light pattern on the E6 light - tall and narrow and more than adequately lighting up the pitch-dark trail before me. This is a light to see with, not just be seen.

Here I am going head-to-head with the light rail line - damn did I blind the driver that night:

And finally here is my dynamo hub and light equipped cross-check before the
Kölner Dom in Köln, Germany (the E6 is handlebar mounted in this photo) - this photo doesn't show the hub or light very well, it's more of a bragging rights thing:

The one downside to the E6 is the fact that the halogen bulb only lasts for about 100 hours of use - which is actually a lot more than it sounds, I turn mine off during daylight hours. Keep a spare bulb with you and you're fine - they're about 4-bucks each. Replace the bulb when you notice the light output a bit fainter than you remember it - it also gets a bit yellower with time, that's also a good indication. The E6 also does not have an internal capacitor, which means when you stop, the light goes out. I don't see this as an issue, as when I'm stopped it's usually because I'm hiding from the man, but there are other lights that have a capacitor driven LED backup light helping to make you visible to others, notably at traffic stops.
People often remark to me - "man, that thing must really slow you down with all the drag it creates." Yeah, but have you seen my legs? My quadriceps femoris are quite truly enormous. For the rest of you less physically endowed individuals (and I look down upon and mock you when given the opportunity,) the drag has been compared to either a 1- or 5-foot vertical climb for every mile travelled, when the light is switched off or on, respectively.
Here's an efficiency test from (Vintage) Bicycle Quarterly for those of you who care - I happened to study my options for about a year before plunging in - this VBQ comparison pretty much summarizes up most of the other articles I read during that time.
There are other dynamo lighting options available, notably the Inoled +10 and +20, which use an LED vs. a halogen bulb in the E6 - the difference between the Inoled +10 and +20 being a 1 or 2 Watt bulb, respectively, and of course the associated price difference. I can't speak from a great amount of personal use from the Inoled - but a few people I know, including my brother, have the +20 model. I've ridden next to my brother - him with his +20 and me with my E6. The +20 has a wider and shorter light output pattern, while my E6 generates a taller and narrower light patch (see above photo.) Also, the +20 hits peak brightness at walking speed, while I have to reach about 10mph (16kph) to gain peak brightness with the E6.
If I had to do it again, I would likely get the Inoled +20 - but wait...
There's a new light coming out! Introducing the
Edelux, made by
Schmidt in Germany. This E
delux puts out more than 3x the light intensity, based on Schmidt's own data, as compared to the E6 model, which up to now offers the highest illuminance from a Schmidt light source.
Where to you get a Schmidt hub or light, you ask? Well, if you're in the Minneapolis and surrounding areas, you should go straight to
Hiawatha Cyclery without passing go - that's where I got mine more than 2 years ago. You could also go to
Peter White, but he probably already has enough business as it is.
Please remember, the US dollar is currently about worthless as compared to the Euro, so this lighting system is in no way an inexpensive accessory - something about the US taxpayers currently spending
$5,000USD each second in iRaq. Me? I'll wait before buying a second Schmidt, which is one component my steamroller feels naked without.
edit-here's one more photo showing how far out the light can be adjusted. The ambient lighting is relatively bright on the trail here, not to mention the reflected light from the snow, so the patch of light doesn't look too impressive in this photo.
