One of the most important things that I carried in my backpack while I was traveling was my international electricity converter. The plug-ins in the States are different from everywhere else around the world. South America's is different than Australia, Australia's different than Asia, and Asia's different than Europe's.
I can't tell you how important the converter was to me on my trip. I carried a couple of items in my backpack that needed to be charged at least once a week. The most important being my Ipod, which I used everyday multiple times! The battery on my Ipod would only last about 2 days without being recharged. I also needed the converter to charge my digital camera battery (which lasted about a week) and my cell phone.
I bought my converter at a Best Buy store. The one I bought looks very similar to the picture above (picture from familyonboard.com). You can also buy them online from Amazon.com, and sometimes you can find pretty good deal on them. I bought my converter for about $12 if I remember correctly.
The converter is very simple to work. The converter is the big piece pictured above. You then plug in the cord from your cell phone (or whatever you need charged) into the converter. Then you take the converter and use one of the attachments that fits in the country's electric plug you are in.
The converter that is shown above does have one downside. It has so many pieces that throughout my trip I started to lose pieces or break pieces. By the time I had been traveling about 6 months I finally threw it away because I had lost the attachment that made it work in Asia. I then had to find other travelers that had a converter that I could use.
When the time I arrived in Barcelona my cell phone, camera, and Ipod were basically dead, because I couldn't find a traveler that had a converter. I finally broke down and bought one that was way better than the one I bought before my trip. This new converter was one piece that had all the pieces attached to it. I bought it for 20 Euros. I thought it was expensive, but it was well worth the money! You can also buy this online through Walk About Travel Gear for $20. I would suggest this converter over the one that has a lot of pieces!