Plugins are very easy to make and register. Here's a simple one that does nothing but support installation and uninstallation. All plugins need to do at least this much.

namespace MyNamespace{
  public class SimplePlugin: IPlugin{
      public IPlugin Install(Configuration.Config c) {
          c.Plugins.add_plugin(this);
          return this;
      }

      public bool Uninstall(Configuration.Config c) {
          c.Plugins.remove_plugin(this);
          return true; //True for successfull uninstallation, false if we couldn't do a clean job of it.
      }
  }
}

You can install your own plugin just like any other - make sure it's referenced in the project, then put <add name="MyNamespace.SimplePlugin"/> into the <plugins> section of web.config.

Or, you can install it during application start using new SimplePlugin().Install(Config.Current);, just like any other plugin.

In V4 and later, you must specify the fully qualified name in web.config, i.e. MyNamespace.MyPlugin, MyAssembly.

See the Gradient plugin for an example of a short, yet very capable plugin.