DNA origami is a powerful method for constructing DNA objects. It involves taking a long, single-stranded DNA backbone (usually about 7000 bases in length) and forcing it to adopt an arbitrary shape using hundreds of short, single-stranded DNA oligonucleotides (each usually 20 to 50 bases long).
caDNAno is open-source software based on the Adobe AIR platform for design of three-dimensional DNA origami nanostructures. It was written with the goal of providing a fast and intuitive means to create and modify DNA origami designs. You can learn how to use it, download a copy of the program and some example designs, or even modify the source code.
The software makes heavy use of several fantastic open-source libraries and resources, especially Papervision3D for 3D rendering, Michael Baczynski‘s AS3 data structures and tutorials, the Tango Desktop Project for icons, and the Blueprint CSS framework for this website.