* Website that has a edit half and a live-preview  https://asciidoclive.com (found by Lucas P)

* IntelliJ has an AsciiDoctor plugin that allows to basically do the same (see below)

* AD syntax http://asciidoctor.org/docs/asciidoc-syntax-quick-reference/#source-code

* AsciiDoctor home page http://asciidoctor.org

* AsciiDoc writers guide http://asciidoctor.org/docs/asciidoc-writers-guide/

* Editing AsciiDoc with Live Preview http://asciidoctor.org/docs/editing-asciidoc-with-live-preview/

* (Start of a) Eclipse Asciidoctor plugin http://marketplace.eclipse.org/node/2162366

* Netbeans plugin https://blogs.oracle.com/geertjan/entry/asciidoctor_netbeans

As Mike mentioned, you can go to GitHub, click on a .adoc page and then use the pencil icon

Directly edit + preview

And then commit / open a PR ( I think commit is only enabled for repo collaborators, everyone else gets PRs)

IJ AsciiDoctor plugin: