With the proliferation of mobile communications, you'll need to adapt the the electricity you find on your travels, whether converting your power from the US 110 volts to European 220, or getting power from your rental car, ferry, or airplane.
Power your laptop or recharge your music player when you travel. This little adapter allows you to plug in to your car's cigarette lighter or an airplane's power system. Comes with plug adapters and supplies up to 70 watts of power to your devices in a small package.
Guaranteed laptop compatibility allows you to recharge your stuff from just about anywhere. Surge protection included. Like the IGo above, allows you to recharge multiple devices. DC and AC inputs.
If your appliance will run off 220 volts (check your power brick, it should say), then all you need to do is to convert your two pronged plug into a European configuration. This will do for travel in most of continental Western Europe.
Need a plug adapter for, say 150 countries? Well, this one works in the USA, UK, Europe, Australia and many other places. Note: this device does not convert 220 volts European to 110 volts, it merely has a variety of plugs in a single adapter to allow you to connect your appliances to sockets used in other countries. Check your appliance or its power brick to see that it works at European voltages of 220 @ 50 hz.
This Belkin plug converter kit has separate plugs for each region, as well as plugs to allow you to plug a telephone into European lines (although these are becoming less useful as Europe adapts US style phone plugs). The Belkin kit allows you to take the adapters you need for the countries you're visiting, while the Macally, above, fits all the plugs into a single unit. Same voltage warning as above.
This is the device you need to "step down" Europe's 220 volts to American 110 volts. It will handle up to 1600 watts, which makes it useful even for high-wattage devices like hair curlers and dryers, but check that yours doesn't exceed the 1600 watts, or you'll be very, very sorry.