Nova Launcher Betas include new features and fixes before they are released to all users. However they also are not as well tested and may not be as reliable. Betas are best for people who don't mind when things temporarily break a little bit.
Betas are distributed either via Google Play by opting in as a tester, or directly from this website.
To opt-in for Nova Launcher Betas from Google Play:
Installing betas does not require uninstalling your current version of Nova Launcher. Betas are compatible with your existing Nova Launcher Prime license and you will not lose Prime features by installing a beta.
For betas outside of the Play Store, install the latest Nova Launcher APK and enable automatic update checks and update from within the app via Settings > Check for updates > Menu > Beta
5.1-beta6 Apr 21, 2017
5.1-beta5 Apr 12, 2017
5.1-beta4 Apr 4, 2017
5.1-beta3 Mar 30, 2017
5.1-beta2 Mar 23, 2017
5.1-beta1 Mar 23, 2017
5.0.3 Mar 6, 2017
5.0.2 Feb 23, 2017
5.0.2-beta5 Feb 22, 2017
5.0.2-beta4 Feb 20, 2017