I use a Light & Motion ARC, which has a 13.5-watt high-intensity bulb and a rechargeable NiMH battery that's designed to slip into a jersey pocket or small lumbar pack. It's all I need—incredibly bright, with both an excellent forward spot and enough peripheral illumination to ensure you always know what's around you. The one downside is that the high-intensity light puts out a very "cold" blue light, which tends to turn everything gray. So sometimes differentiating roots and rocks on the trail is a little tricky. But then again, it is night. I recently had to send mine back to Light & Motion to have some work done on the ballast, but otherwise it's been a tough, reliable light. And battery life is great—I've never come close to running it all the way down, even on rides of two hours plus.
I'll concede, the ARC is a bit pricey at $410 (www.bikelights.com; several online bike stores have it for $370 at the moment). But it's well worth the extra green. For a time the guys I rode with each had two "regular" lights comprising headlamps and bar-mount lights. And the ARC outshone them all. A worthy competitor is Niterider's Storm ($439; www.niterider.com), which has a slightly more compact battery but otherwise very similar specs.
If that's rich for your budget—which is understandable—Niterider's Digital Evolution ($230) is a good choice. It lacks the light-cannon properties of the high-intensity bulb on the ARC or Storm, but it's still admirably bright, with three power settings for maximum battery life during a ride. I used one of these for several years on nighttime road rides, and was always happy with it. Coming down the scale even further, you might look at CatEye's Compact Opticube for a reliable yet slightly weaker lamp that won't do you wrong for less technical nighttime trails ($35; www.cateye.com). It features two settings (beam or strobe) and, better still, can be fitted to a helmet for your nocturnal sledding needs. To beef up the illumination on singletrack without breaking a C-note, combine with a CatEye Power Opticube ($50) fitted to your handlebars.
Finally, if the sledding portion of your question is really the goal, consider an out-and-out headlamp, something like Petzl's Tikka Plus ($36; www.petzl.com) or their burlier MYO 5 ($77). These are mountaineering lamps, but should cast enough illumination for sledding runs, as well as lead the way back to your nighttime stash of Stoli (which I can only assume is the inspiration for hitting the backwoods with a sled once the sun sets).
Be safe—and have fun!
Check out Outside's 2004 Buyer's Guide for more essential mountain-biking gear.