Docs for the API's can be found at https://www.adiirc.com/docs/.
You will notice there are no GUI API's yet, i'm open for any API requests for this.
If you want to play with this, but don't know what to do, you can download a sample plugin in Visual Studio C# Express format at https://www.adiirc.com/plugins/MyPlugin.zip
You can download Visual Studio Express at https://www.microsoft.com/express/download/
Plugins can load and use any DLL's including referencing all .NET frameworks DLL's and do P/Invoke, just like a regular .NET program.
If you get a "System.NotSupportedEception: An attempt was made to load an essembly from a network location" error, try unblock the dll file as shown here here
Look forward to see some creative plugins, and please ask if you have any problems.
See also Plugins.