A Word about Gtk:
GtkRadiant is using the Gtk toolkit for all the user interface. There's a specific page for the Gtk on win32. If you are going to write a plugin for GtkRadiant, we recommend you use Gtk, but it's not a required feature. You can write plugins using the native windows API or even VB. Don't hesitate to ask for help on the plugin mailing list or to drop by on irc.telefragged.com #qeradiant

Plugin basics:
main features available to plugins:

• Manipulate MAP data: read and write brushes, patches, epairs and entities
• Override the BSP menu to process your own building commands
• Read / Write project settings epairs
• Use OpenGL to draw in the 2D/3D view, or in your own plugin windows
• Use listeners to catch events in Radiant (like a change of texture or brush select / deselect)
• Use listeners to listen for window events (for mouse interaction in the 2D view)
• Create new plugin entities that show up in the Radiant window, which you can draw yourself and handle the user interaction
• Use the Radiant internal parser to hook your own MAP format changes for plugin entities
• Access polygon and texture information on the current selected face
• Override the texture / shader code to provide your own texture formats and shader system
• Add new surface properties, and change the MAP format accordingly

• QERPlug_Init is the first entry point called, used for initialization and sending back the plugin name that appears in the console
• QERPlug_GetName returns the name of the plugin as it appears in the plugin menu
• QERPlug_GetCommandList returns a list describing the items in the plugin submenu
• QERPlug_GetFuncTable returns the address of the _QERFuncTable_1 of the plugin. After Radiant got the pointer to the function table it will fill it and the plugin is able to call into the editor.
• QERPlug_Dispatch is called when the user hits a command in the plugin menu. The plugin is then free to process.

NOTE: some function tables are used by Radiant and must be filled by the plugin (ie. they work in reverse compared to usual ones). In this case the plugin must export an additional entry point int WINAPI QERPlug_RequestInterface( REFGUID, LPVOID );

virtual classes:
some stuff is better represented by an abstract C++ class. For those with COM knowledge, it's just a very lightweight COM way of doing things. These classes have very basic reference counting through IncRef() and DecRef() .. you are supposed to increment or decrement the reference count each time you store or release a pointer to these classes, otherwise they could get unexpectedly erased and you would be very very sorry. The IListener class in IMessaging.h can be implemented by the plugin and handed over to Radiant to listen for events. See TexTool for examples.
 

More stuff:
If you need more information or if you want new features for plugins, see the source code for TexTool and Curry. TexTool source comes with the plugin SDK, and Curry source is available from their web site. You can browse it online from the project page. PrtView has neat code samples on how to draw in the 2D views or camera window. These two plugins are under GPL licensing. The dedicated place to talk about plugin coding is the plugins-coding mailing list ... see you there. You may also get an updated list of the plugins for Radiant on our dedicated web site.
Update: you can also have a look to the pk3man source.

Conclusion:
this document is intended as a quickstart for potential plugin writers. I hope it meets it's objective. It's not a complete and systematic documentation, I doubt there will be one ever unless someone decides to do it. Feel free to send me contributions to this document. Send feedback to me.