Sample Plugins
A sample Maven-based Workspace Access plugin can be found here: https://github.com/oxygenxml/sample-plugin-workspace-access.
There is also a sample project which contains various JavaScript-based plugins: https://github.com/oxygenxml/wsaccess-javascript-sample-plugins.
The Oxygen GitHub site contains lots of open-source plugins (https://github.com/topics/oxygen-standalone-plugin). Most of these plugins are of the Workspace Access type.
You can also find a variety of other publicly-hosted Oxygen plugins in the Public Hosted Oxygen Plugin and Framework Projects blog post.