Platypus Gravityworks Filter
This filter is an excellent option if you want a lot of filtered water fast, and with very little effort. GravityWorks, as the name implies, relies on gravity filtration, and comes with two 4-liter reservoirs. Both have attached loops so you can easily hang them from whatever happens to be nearby. One reservoir is dedicated exclusively to clean, filtered water (for obvious reasons). In fact, the only way the "clean" bladder can be filled with water is through the filter itself: a design which prevents you from accidentally putting dirty water into both bags and contaminating your entire apparatus. A quick backflush at the start of each filtering session takes seconds, and will extend the lifetime of your filter. Just hang the bags, and in about two and a half minutes, you'll have 4 liters of filtered water.