For U.S. and Canadian travelers, electrical converters are not necessary because Mexico operates on the 60-cycle, 120-volt system; however, many Mexican outlets have not been updated to accommodate three-prong and polarized plugs (those with one larger prong), so to be safe bring an adapter. If your appliances are dual-voltage you'll need only an adapter. Don't use 110-volt outlets, marked for shavers only, for high-wattage appliances such as blow dryers. Most laptops operate equally well on 110 and 220 volts and so require only an adapter.
Steve Kropla's Help for World Traveler's (www.kropla.com) has information on electrical and telephone plugs around the world. Walkabout Travel Gear (www.walkabouttravelgear.com) has a good coverage of electricity under "adapters."