Keep in mind that it's not just the voltage that matters. Different countries also have their AC power cycling at different rates.

Most 110-120v countries cycle their power at 60hz.
Most 220-240v countries cycle their power at 50hz.

This doesn't matter for simple elecrical devices like lamps, blow dryers, and razors. For those devices a basic voltage converter will do the trick. But you can actually fry sensitive 110v/60hz electronic equipment by plugging it into a 220v/50hz outlet with a voltage converter that doesn't alter the AC cycles.

As stated above, the VAST majority of laptops and digital camera/camcorder chargers, and many cellphones and similar devices have power bricks that already support the full range of worldwide current (110-240v, 50-60hz), so all you need is an adapter.

But if you have anything that's particularly sensitive and not easily replaced (especially things like stereo components or scientific devices), I would be extremely wary of plugging it into a foreign outlet with anything less than a full-blown $100+ transformer that converts both voltage and the rate at which the current cycles.