Those are great for whitebox-type stoves. Basically you cut the bottle and take the top part and put it upside down in the bottom part to create a double walled stove. Before doing this put some notches in the top for the fuel to flow through to the space between the walls. Put some holes in the outside of bottom part (the outer wall of the stove) for the flame to come out.

I saw a video online at one point showing the whole process but I can't find it now. Minibull design sells a stove just like this. Here's a video.

http://www.youtube.com/watch?v=GelDYw1HjWU

Have fun.

Heber