SteriPEN Handheld Water Purifier
We can't speak highly enough of this product for someone who likes to travel, particularly to more adventurous places where easy access to clean water isn't a given. This little device uses UV light to kill 99.9% of bacteria in up to a litre of water in just 90 seconds - including common travel bugs such as giardia.
Unlike other chemical water purification treatments it doesn't alter the taste of the water at all. It can be used up to 8,000 times and is easily rechargeable via a computer, mains supply or separate portable solar charger. With one charge it can treat up to 50 litres of water.
There is also an even smaller ultra compact version.