A Moodle Nightly is built automatically every day from the most recent code, and is available from the Moodle.org download page. It can be useful if you don't have CVS, but you want to try the latest features. It can also contain bugs, since they may not have been well-tested yet.
It is helping Moodle adoption as part of a greater project of developing OSS e-learning application software for deployment throughout New Zealands education sector. System integration of portal website framework, lcms, content creation tools and modular toolkit.