Nightly Builds are releases of Liferay that are automatically created every night by the automated build system. By using these releases you can play with the latest and greatest Liferay trunk every night without needing to download the entire source and compile it yourself.
The nightly builds can be found here:
Be aware that these releases are created out of the latest source code from trunk so don't expect them to be stable and of course don't use them in production.
Having said that this is a great way to stay tuned with the ongoing development and provide your feedback through the forums.
Note: If the link above shows an empty directory it might be because there was a problem with the build that night. Please check the next following days and it will probably be solved.