Base26 by Toxi, built in Processing.

Processing (created by Ben Fry and Casey Reas) is one of the most significant new tools to address the needs of artists and designers experimenting with code. In recognition of its impact Processing was recently rewarded with the Golden Nica in the Net Vision category of the prestigious Prix Ars Electronica. The project was inspired by previous work such as Design by Numbers, and is free for download and use.

Processing is a programming GUI and simplified programming language offering 2D and 3D graphics, video processing and serial communication with physical interfaces in a simplified syntax that is friendly to both beginners and experts. The language is built on top of Java, so expert users have access to object-oriented programming and other advanced functions. Processing is meant to be a tool for sketching and learning about computational media, and is currently in use in dozens of schools around the world. The Processing community now counts thousands of users, many of whom are exchanging code and new libraries on the Processing discussion forums.

The software was recently been released in a Beta version, which introduces multiple file editing, OpenGL support and a new plugin library system. It also marks the transition of the project to Open Source status (various parts under the LGPL license and GPL licenses), meaning that anyone can download the source code and become a potential contributor.

Download it from Processing.org and check out the exhibition section for sample projects.