I recently picked up an Aquamira Frontier Pro Ultralight Water Filter for use in my go bag. There are similar to the popular "survival straws," but good for filtering more water and with some handy features.
They are small in size, light and easy to pack. The filter is good for filtering 50 gallons of water--in the right conditions, that's almost two months worth of water for one person. The Frontier Pro is also quite versatile--you can drink through it like a conventional survival straw, hook it into to a Camelbak drinking tube or screw it onto any standard sized water/pop bottle top. You can suck the water through the built in bite valve or set it up as a gravity filter if you've got two containers to work with. Pretty cool and versatile--very smart design.
The filter has an integrated cap to cover the bite valve, keeping it clean. Like all filters, the Frontier Pro will not remove viruses. You'll need some kind of purification method (chemicals, UV or boiling) to get rid of those if you're concerned about them existing in the water--dead bodies upstream, people crapping in the water, etc.
For $20, the Frontier Pro is hard to beat. The size and weight make it incredibly easy to add to any pack--your EDC bag, BOB, hiking pack, whatever. I keep a small, collapsible 1-liter Platypus Sport Bottle with mine, which makes sure that I have a way to carry water with me. The Platypus adds a little bit of additional bulk--I'm looking for an alternative but haven't found anything better yet.
I'm a huge fan. The Frontier Pro is a compact ultralight solution for pretty much all of your kits--anything bigger than an Altoids tin, at least. They're also cheap enough that you can afford to put them in all of your kits--your EDC bag, your GHB, your BOB and the kits of your family member. That's good insurance in my book, and a lot better than in sinking all of your cash into one whiz-bang water filter that may break or be forgotten about when TSHTF.
Check out the Aquamira Frontier Pro Ultralight Water Filter >