Hudson is a continuous integration server that will watch a source code repository and automatically build and test the code whenever it is updated. There is an interesting plugin that lets it integrate with Amazon EC2. If you need more server resources to build and test the software, Hudson will simply bring a new instance up on Amazon, use it and then shut it down.
Since the actual source code in a project is typically not going to require much bandwidth to move around, this would work a lot better than trying to use EC2 for other on demand applications–like video rendering that are both processor and bandwidth intensive.
