The API for plugins - Mojos - development.
Java 5 annotations to use in Mojos
The Maven Plugin Testing Harness provides mechanisms to manage tests on Mojo.
Atlassian REST Common
Maven Plugin Testing Mechanism
SonarQube :: Plugin API
A library to give systems the ability to have plugins, make them more pluggable and hence add pluggability (it's late - that makes sense in my head).
Atlassian Plugins Web Resources