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: ppa:nightingaleteam/nightingale-nightly
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.