For shorter commutes, I'd go with a flashlight up front. Here's my fav outfit for that:
http://www.bikeforums.net/showthread...=#post14043976

For longer commutes, I'd decide how much to spend. If < $100, I'd get a light off eBay, search for 1600 lumen bicycle headlight. Note that the batteries are not waterproof, and the beam is likely to be too tight (too spotty) - ebay search for "magicshine lens" - I use that lens on a $35 ebay light and it works quite well all winter.

If you want to spend more money, go for name brands.

If you get a magicshine sort of headlight, you can use the MagicShine taillight which runs off the same battery pack as the headlight, so that's nice.

For self-contained taillights, the Cygolite Hotshot is pretty nice. The Serfas Shield is my favorite but it's expensive.

Since I personally insist on having two front and two rear lights (the rear ones both on because I won't know if one goes out, the front one in reserve) IMO you'd do well to buy something like the flashlight and a Cygolite at first and see how it goes for you. If you decide to go for something bigger or name brand, you will have the flashlight as a backup in case you forget to charge a battery or something flakes on you.