Open source is a well-known style of software, but because the open source community has definite ways of doing things, there are lessons to be learned that are applicable to rapid development.
In an open source environment, there is usually one person who maintains the software. Others can make coding changes, or they can add code to a basic system, but it is done through patch files fed to the maintainer. The maintainer reviews the changes or additions, approves them (or not), and adds the changes to the basic system as appropriate. The key point in this approach is that only one person can make changes directly to the system, which protects the integrity of the system and facilitates configuration and version changes.
All the light methodologies have an adaptive nature to them. One major benefit of these techniques is that the design can be changed or fine-tuned as it evolves. One of the problems in software development is that the value of the software cannot really be assessed until it is complete and in use. It is often difficult, or impossible, to completely define the system requirements before starting an IT project. So using an adaptive process allows the developer and the customer to jointly develop the requirements as the project progresses.