Hi - the things I found most useful in the recent blackout:
--Carrying a flashlight (actually, a couple). I had an Arc LS, which I used to help guide people out of a dark subway station, and an Arc AAA that I lent to someone. Small, bright enough to find your way around (even a keychain light would suffice in a pinch) and preferably LED, because they last longer and won't suddenly reach the end of the bulb life or break in a short drop.
--A radio - I had a Walkman-style radio with me, and was able to find out (as the story developed) how widespread the outages were, and that power wasn't likely to come back soon. I had to persuade some of the people waiting on the subway platform for the next train that there wasn't going to be one. Good for any other available info, rather than being left to guess what to do, where to go (or where to avoid).
--Extra batteries (which I didn't have then, but do carry now) - a few AA or AAA batteries are tiny, stow easily and greatly increase the useful life of whatever electrical gear you do have with you. My radio's battery life was on its last legs by the time I got home.
--Water. I keep a bottle in my backpack/bookbag. The blackout was on a hot day, and many stores just closed, others sold out of water, and still other entreprenuers were selling water at double and triple the normal price (this was not the common action that I saw, but some did).
At home, I had some water in 2-liter bottles stored in a closet. We still had water in our building, but after this, I'll be getting some 7-gallon jerry cans to keep in the basement in case something like this happens again. Search this site for storage recommendations. Also, another suggestion I got from this site was to keep some 2-liter bottles of water (about 2/3 full to allow for expansion) in the freezer. If power's out, some can be transferred to the fridge to help keep it cool so food doesn't spoil as fast, and when it melts, you've got drinking water. Simple and ingenious).
--Home lighting. I had lots of flashlights, enough to lend one to a neighbor, and was set for batteries. But the thing I found was, once you're home and settled and not hunting up stuff in the dark, you want an area light to give comfort and practical lighting. We spent a bit of the evening on the couch, my wife with a flashlight tucked under her chin, blasting light at the magazine she was reading. I'm going to add a couple D-cell battery-powered LED lanterns to the mix - they're rated at 40 hours battery life at full power, and its output is adjustable.
Discussion of LED lantern

We had candles and some decorative oil lamps, but neither's the best idea with a toddler underfoot, and the chance of accidental fire still looms (the majority of the fires the night of the blackout were said to have been caused by candle use).
-- Food. We had a normal pantry's worth of foods, plus a few MREs stashed away, and wouldn't really have had a problem in that regard. Cooking would have been a hassle - I have Esbits and cans of Ecofuel, but both were a bit too much hassle to dig out for one night's use. I will probably add a small charcoal grill and keep a bag of briquets in the basement.
-- Community. As has been mentioned in many other threads and postings, there comes a time when you can help others and/or may come to need their help. Much harder to begin introducing yourself in an emergency when everyone's panicked and in need.
Living in a city can make the sense of isolation in a crowd very distinct. I met several next-door neighbors for the first time when they were barbecuing behind our apartment building the night of the blackout. I knew several of these people in the sense that we realized we lived next to each other and would acknowledge each other with a nod or "how's it going," but we didn't know each other's names. I met spouses of several of them, and their kids - my next door neighbor's son has the same name as mine. It's a small world.
This weekend, we're having our downstairs neighbors - two couples and their kids - up for coffee. Hopefully, we'll get at least to the point of knowing a thing or two about each other, but also we'll trade contact information with them - home and work numbers, e-mail - and hopefully just get the ball rolling on knowing each other and trusting each other. We've already picked up mail for our downstairs neighbors when they went out of town for a week - small courtesies mean a lot, especially in a city that tends to isolate people. People also don't want to intrude, because with the necessary crowding of a city comes the very strong sense of boundaries and dislike of people encroaching on your privacy.

Thanks to everyone here at ETS for ideas that really made a difference for me on the day of the blackout.
Dave