Since Codemonks.ORG is full of hikers, this is meant to be a compendium of information.
(Someone else should fill this in)
Very useful for...
Long Distance Backpacking
Pacific Crest Trail: http://www.pcta.org/
Personally, I like making my own version of pemmican, and a form of rice and bean gruel that cooks up quick.
Amongst all of the hype, there is this link: http://www.yosemite.org/naturenotes/Giardia.htm
I still like to have a filter, because even lacking giardia, lakewater is not always pretty.