I actually just made a map-editor example since someone asked about one a little while ago. It's strictly VB.net though, so the actual mapview-usercontrol displaying the map relies solely on bitmaps drawn in a graphics object. This works pretty well as an example (32x32 bitmaps with multiple layers on each map-tile of terrain, resources, unit and buildings), but for a commercial solution, you will need XNA (C# only I'm afraid) or DirectX at the very least IMHO.
The Editor currently supports thin pencils, thick pencils, flood fills and eraser of terrain, resources, unit and buildings either by-click or continuous drawing. I could add a link if you're interested.
At the moment I'm working on extending the MapView control to DirectX using elevated terrain and making an easy-to-use 'make-your-own-map-game' type of library with editor to distribute as freeware here. Since creating interesting maps and displaying/editing them seems to be the biggest hurdle when trying out ideas.
Depending on the structure of your maps, modifying my code may be a quick alternative to actually coding your own editor.

Regards
Tom