You've opened a can of worms almost as full as the can related to "what tires are fastest"...  
 
It sort of comes down to how good your night vision is, and your budget.  You can spend anywhere from $50 to $500, like most things.  But there are both diamonds in the rough, and some pooh-pooh.
 
Also, there's a difference between a light intending to GET YOU SEEN, like a blinking Cateye or similar, vs. a light you intend to SEE WITH.  I will assume you're talking about the latter.
 
1)  Don't bother with the Cateye style LEDs -- not enough illumination;
2)  I started with a 10W/20W halogen set, from Performance -- it worked OK, but only had 2 hour battery life, and the light would dim significantly in the last 30 minutes;
3)  I wound up purchasing an MR11 HID light before the TTTT, from www.trailtech.net, with a 5 hour Li-ion battery, for $225.  They make 'em for motorcycle/motocross.  However, this was in mid-2007, and their price is now $300 --  I guess word got out.    But it's the equivalent of a prison searchlight, and for my marginal night vision, it's great, although a bit heavy.  
4)  I've heard/read good things about other lights from Nelson, et al -- I'm sure they'll chime in!  
 
You can probably borrow and "test light" before buying, to see what works for you!!!!
 
Just be sure to BE SEEN, either through flashing lights or reflective gear.....
 

Reflective gear is great. It is always on when you have a light pointed at you like a headlight and you don't have to worry about batteries. A light has to be pointed at/near you for it to work though. Riding on curves or in areas with lots of intersections means you really need lights.
 
The trick is it really really depends. Sometimes I get by with just my little fenix L2D CREE helmet light on low. Yup on low. When there are no cars out, no streetlights and the moon is not out or bright you don't need much light to see.  
 
The more light from other things there are the more light you need to see. At least until there is enough light to see without your lights then you just need something for people to see you  
 
I use a homebrew led light on the front of my bike which has a nice wide lens. It'll light up the sidewalk on both sides of the street in addition to the area in front of me. This allows me to see people/animals about to shoot out in front. A helmet light can also do that but my position on the giro make a helmet light bounce alot of glare off the handlebars/riser/cyclocomputer. I used to have a dinotte 200L in front and it worked great to see where you were going and allow people to see you it just didn't give me enough overall view of the world since I can't use a helmet light that much anymore.
 
On the rear I have Dual Dinotte 140Ls and Dual Superflashes. Mostly for redundancy and so I stand out when I'm playing with tractor trailers on my commute.
 
 

As mentioned above all kinds of lighting solutions.  For commuting the last 9 years my lights have evolved through the cheapie cateyes through home bew MR16 halogens to ultimately fancy super bright LED flashlights.  My requirements also evolved to what it takes to ride all night in randonneuring conditions, and there is a real tradeoff that has to be made between brightness and 12hr running time and weight.  Generator hubs are very poppular for this still but are very expensive (I have 3 for different wheel sizes) but I now feel my little Fenix L2D CE 2aa light with extra batteries is better.  Its proven reliable, and 2 of them, with spare batteries are still lighter than the generator hub and doesn't create drag.  Like Mark said as well, for night riding reflective gear is more important than lighting, as it will be the first thing a driver sees from the rear or side of you.
 
Take Care,
Nelson.

This was recently posted on Bentrider...  670 lumens!!!!
 
 
http://www.bentrideronline.com/messageboard/showthread.php?t=43327&referreri d=4000