I agree with the previous reply.... it sounds like you're referring to a Plug Adapter and not a voltage converter, which steps-down the voltage from ~230 to ~115 VAC. These can't convert the 50 Hz European frequency to the 60 Hz North American frequency, but that's not an issue in most cases.
The Swiss plug adapters shown in the first reply should work fine. These are the three pin, grounded plug adapters with the centre (ground) pin slightly offset from the other two. This is different than Italian grounded plug adapters which have the three pins in-line.
If you're just going to be charging mobile phones, cameras or other devices that only use a two pin plug, you could also use the ungrounded Euro plug adapters, such as THESE MODELS. It's a good idea to take several, as they're small and easily misplaced.
One important point to note is that it's extremely important to check the input voltage requirements for EACH device you'll be using in Europe, to ensure that it's designed for operation from 100 - 240 VAC. This information will be listed on each charger somewhere (you may need a magnifying glass).