Try and search for SDKs of some large software. Usually those would be programs for creating content - audio, 3d modelling, 2d drawing, etc. Every major vendor has a plugin architecture that quite obviously leaks implementation details. So stuff like Adobe Photoshop, Autodesk 3dsMax, FL Studio. All these have public SDKs that you can download, explore and write plugins for. You can probably think of some more programs that support third party plugins.