The Katadyn Hiker Pro Filter is a very popular water filter that filters water quickly (1 litre per minute) and easily. The filter includes a field maintenance kit to help with any issues that may arise in the field, a cleanable filter protector, which helps to extend the life of the filter (although it does slow the pumping process). The filter is easily cleaned by swirling it in water.
This filter physically removes particles, protozoa, and bacteria down to .3 microns in size, which includes Giardia, Salmonella, and Cryptosporidium. The filter is composed of two filtering mechanisms, including a glass-fiber element, and an activated-carbon core to absorb chemicals and pesticides.
Several quick-connect fittings make it easy to fill up hydration packs, as well as an adapter to fit most water bottles.
After working with several filters during my backpacking history, I have settled on the Katadyn Hiker Pro, for it's speed, the lifetime of the filters, and its ease of use. We used this filter for the vast majority of our Pacific Crest Trail Hike, as well for our hike of the Colorado Trail.