Previously there was an easy way to download the nightly version (link comes from e.g. https://forum.getkirby.com/t/kirby-2-2-public-beta/2272). That was fine.
But since some time I only get “kirby-nightly-2.2.1-20151118133433.zip”.
You can download the Kirby/Tookit and Panel develop branches directly from the GitHub repositories, even without having Git installed. Just switch to the branch you want to download and click on the “Download ZIP” button. Currently, there is no develop branch of the Starterkit.