My number one device in my bug-out bag is my LifeStraw Personal Water Filter. The LifeStraw is only $20 and will filter up to 1,000 liters of water down to 0.2 microns removing 99.999% of water-borne bacteria. This $20 straw can supply a single person with over 260 days of clean drinking water. Each of my family members are equipped with one of these.
The next water purifier I keep with my main supplies in my house is the SteriPEN Traveler 3-in-1 Water Purifier. This purifier uses UV light to kill organisms. At $42 for 3,000 16-oz treatments, it's a good backup to have in addition to the LifeStraw. The only thing I do not like about the SteriPen is it relies on AA battery power where as the LifeStraw does not.
If you are aiming to supply safe water for more than yourself, I highly recommend the LifeStraw Family Water Purifier. LifeStraw has introduced the new LifeStraw Family 1.0 Water Purifier for $75 which will filter up to 18,000 gallons of water, enough for a family of four to stay hydrated for 3 years. No power is necessary for this purifier as it uses gravity to filter the water. So, three years of clean water for $75 and you don't have to store containers of water around your house? Folks, this is a no-brainer purchase if you ask me.
Lack of clean water will be the number one killer after a disaster so please do not skimp in this area.
Keep on Prepping!