DynRPG is a plugin SDK for RPG Maker 2003. “SDK” means “Standard Development Kit”, and in this case it means that everyone can create his own extensions to the RPG Maker software now, as long as he knows how to use C++. DynRPG not only offers the average programmer access to the world of “patching”, but it also provides the basis for far more powerful extensions than all “patches” made for the RPG Maker before.
DynRPG works using plugins. Plugins can be written in C++, they will be loaded when the game starts and they are notified of certain events (like the drawing of a new frame, the writing to a variable, etc.) and are allowed to react to these events.
For more information, please see the project page.
Update: There are newer versions than this version 0.14a! There is a never-fully-released version 0.20 here (which is nonetheless the quasi-standard now) and an improved fork by PepsiOtaku (0.30+) here.