I have always wanted to make a cloth simulation program. The idea of a grid of points controlled by a bunch of springs just seems so simple to implement with such interesting results. Thanks to people much smarter than me, like Thomas Jakobsen, there is a de facto algorithm for it and plenty of tutorials. I've pretty much completed the graphics part of this exercise, which means I have a variable grid that I can draw... now I just need to put in all those equations and algorithms to make it move around.
Here's a screenshot of the progress. One interesting thing in this picture are the colored parts of the grid. I allocate an array for the vertex colors but have not yet initialized it, so its just random data. The colors change a little when I make touch events.
Cloth is pretty much finished now, more pictures and a short video on flickr.