To us, Dead Meets Lead seems like a pretty good start for a mod. It's got a fairly feature-complete editor. It's written in C# and is pretty easy to extend and change. Only problem so far has been that we haven't released the source code, nor the editor.
That changes today. We're very happy to announce that Dead Meets Lead, together with our custom game/graphics/physics/sound engine and editor, now are completely open source. We've also created a new installer which bundles the editor with the game, and also makes it easy to open custom maps in the game.
To get the source code: Visit us at GitHub
Two short disclaimers though; First of all, we're not actively working on this project any more, so don't count on updates and/or fixes. And secondly, the code is pretty much in the same state as when we released the game, we haven't really done anything to polish or refactor it afterwards (except a small fix to get maps into a single file format).
Feel free to tweet to #dmlmod if you create some cool maps or mods! (Or post to us here!)