Most countries that use 220V (240V) use 50Hz. But if your appliance is rated for 120/240V and only 60Hz, it may still blow up. For electronics generally you can use a slightly higher frequency alternating current but not a lower frequency than what the appliance is rated for. (Most 50Hz e.g. European appliances other than analog clocks will work just fine on 60Hz and within the range of rated voltages.)

A few devices require a manual switch setting for 220 vs. 110.

Travel tips: http://members.aol.com/ajaynejr/travel.htm