Starting with KSP 0.14, the game supports custom part behaviors called Plugins. Defining custom behaviors allows you to create totally new kinds of parts, such as flight control computers, new types of fuel supply chains, etc. You can then add these to the game in the form of plugins. These DLLs can be loaded on any platform that can run KSP.
READ THE POSTING GUIDELINES BEFORE PUBLISHING YOUR PLUGIN
To use a Part Module Plugin, just drop the DLL into KSP/Plugins. If the folder doesn't exist, you can create it.
First, decide which IDE you will use. Visual Studio or the associated Express Edition works the best on Windows. The others also work on additional platforms, like Mac or Linux.
|MonoDevelop||LGPLv2 / MIT (Free)|
|Nant (Command-line only)||GPLv2 (Free)|
Now, you can get started.