When a major blackout hit New York City in 1977, it resulted in widespread looting, vandalism and violence. In comparison, the massive blackout of 2003 that disrupted electricity to 50 million people across Ontario and eight U.S. states for up to four days was more a severe headache than an occasion to run wild. Still, it brought a new level of awareness to millions -- that being prepared is not so much a crazy survivalist notion as a common-sense necessity.
An uninterruptible power supply (UPS) for your computer buys you a bit of extra run time when the power goes, as well as some protection from surges when the power comes back.
A land line. If you're not purely a cellphone user, consider keeping at least one old school, plug-in telephone around. When the power goes out, your cordless phones are useless but the old plug-in handset on the land line will still work. Land line handsets are still the most reliable way to reach your local emergency services.