All editor plugins that I know of are written in C#, as is the editor. You didn't mean an engine plugin did you? 
No, I did mean an Editor plugin.
I have to admit I downloaded the official plugin classes from the AGS website (and saw only C++), but didn't understand well if they were for the Editor or the engine -- I was all confused.
Like a boss. Like a n00b.
EDIT: I can't find an Editor plugin that comes with its sources in the forum. I don't understand if the practice of using C# for an Editor plugin is natural and straightforward, or if one requires a simple C++-->C# wrapper. (I'm not good at organizing a .Net project, I can only program). First, can you confirm that one only needs
this to code an Editor plugin?
EDIT2: I actually only found
this page using Google. I didn't find it when I navigated in the AGS website, I don't now what's the links combo to reach it :-)
EDIT3: OK, I saw that there is a link to the Editor plugin API in the text at the top of the Engine plugin page, but it's really not obvious, it's pretty confusing if you're used to finding the information displayed in a structured way on the page.
I suggest:
- that the links to the Engine plugins API and the Editor plugins API pages are put side to side on a dedicated "plugins" page,
- that the Engine plugins page states it clearly that it's only for the Engine.
Who's in charge of the website lately? The new design is really cool but it's also time to re-organize the pages, at the moment it's a real mess between downloads, resources, official stuff, non-official stuff, etc.