As this is the current release version, you can download it from the home page.
We are providing nightlies everyday if there are commits in the trunk branch, you can download them to check out new features and develop add-ons or feathers for the next version. Note that nightlies builds are not considered as stable releases, and might cause damage to your profile.
For Ubuntu, there's a nightly ppa available:
We are providing testing builds irregularly, you can download them to test things or debug Nightingale. Note that testing builds are not considered as stable releases, and might cause damage to your profile. Old testing builds are available currently, but you should not use them if a more recent build (testing or release) is available.
If you want to build it on your own, here are some informations for your OS:
Nightingale can be upgraded using upgrade packages. On how to build them, check out updates
In order to build Nightingale's API documentation (also hosted on http://developer.getnightingale.com) you need doxygen and a local copy of the nightingale-hacking Github repo. On Ubuntu-like systems you'd need to run the following command to install all the needed dependencies:
sudo apt-get install doxygen graphviz
Enter the documentation directory and run the build-documentation.sh script.