It claims to filter out 99.9999% of bacteria, and can be reused tons of times (for drinking up to 1,000 gallons!). Plus, with each straw purchased, the company provides clean water to a child for a year. No wonder it's got over 7,700 positive reviews. Check out BuzzFeed's full write-up on the LifeStraw.
Promising review: "This thing is no joke! I took it backpacking in Maine's Bigelow range. When I ran out of the water I brought, I was A-OK with this. On top of one of the lower peaks, I drank from the most disgusting-looking crevice filled with brown scum. I thought, this is going to be safe, but taste horrid. WRONG. IT TASTED LIKE THE FRESHEST SPRING WATER, better than any tap. I'm from Maine, and I've tasted plenty of water right from mountain springs, so take my word for it. One note: it can be a little challenging to use at first. You have to start by soaking it; if it still doesn't work, blow into it a bit. Get one for your outdoor adventures, and one for your bug-out bag, which everyone should have in case of natural disaster." —Amazon Customer
Get it from Amazon for $11.99+ (available in two designs).