Please see https://www.thingiverse.com/thing:3421671 for a new version of this concept.

I explain it all here, but Brian does a much better job in this industry write-up (I am so amazed and thankful for his efforts): http://3dprint.com/15917/3d-printed-water-purification/
Since early childhood I have always wanted to "make the world a better place", a quote from my youth. Now I'm 47 years old (going on 11 lol) and I have the time and resources to do just that. Here's how.
This simple thing is designed by myself to convert a trashed 2 liter bottle into a solar water purifier. You cut the bottle into two sections (cut as shown near the bottom) and insert this 3D printed ring-like object. Then leave it in the sun all day and at the end of the day there will be some clean water in the collection tray of the object.
What makes this invention so likely to actually be used is the following fact. If you give a desperately impoverished person an object with the intention of helping that person survive, what typically happens is the person will sell the object to buy food. Therefore it stands to reason that to help such a someone, what we give them must be basically trash that nobody wants and therefore has no sale value. So the recipient will keep it and use it for its intended purpose, in this case to clean dirty water.
I am giving this invention to the world, meaning whoever wants to take it to the next level is free to do so. I only ask that you tell me if you plan to help the poor with this simple idea / gizmo. Let's end human suffering in every way that we can!
Les
p.s. i made a little video on this concept: https://www.youtube.com/watch?v=l-vKoAXX3QY&feature=youtu.be
p.p.s. i tested the object today (Tue Sep 16 2014) and discovered that it must be sealed to work. If not sealed, all the water evaporates out of the device! More details are in the Instructions section.
this video describes another technique that does not require a 3d printer at all: http://youtu.be/NnqQT287Rw4

First measure the circumference of your 2-liter bottle and put that measurement in the circumference declaration at the top of the OpenSCAD file, then generate a new STL file. Print the ring thing on your 3D printer.
Then cut the bottle in half using a sharp razor knife such as an X-Acto knife or a box cutter. Please be careful while cutting! Next reassemble the bottle by attaching the top and bottom of the bottle to the invention ring.
To operate the device, put some salt water or muddy river water in the bottom section and leave the whole bottle out in the sun all day. By the end of the day there should be some water that evaporated and condensed and dripped down into the inner tray of the thing. Neat, huh?
And finally now that you're empowered to do so, go change the world in the best way that you can, even if it's only spreading the word about this thingie. Together we can make a difference!
Les
p.s. there is an issue related to leakage. As it is, the initial prototype was not sealed so when I tested it, the water preferred to evaporate out of the hot bottle and printed device though some of it did condense and gather as planned. What is needed is a vaseline seal or perhaps the object can be made of NinjaFlex, BendLay, or some other flexible material. There are lots of options for overcoming this unanticipated obstacle. Be creative!