Ratpoison's goal is simple: kill the mouse. Because of this heavy reliance on keyboard commands for interacting with all aspects of the system, Ratpoison has a bit of a learning curve (though that is true of many of the Desktop Environments in this list). Once over that initial hump, Ratpoison is fast, customizable, easy to use and stable as all heck. Plus, you probably won't need to worry too much about setting up any security on your system, because most people won't even know how to open an application if they sit down at your computer. I call this “Security through being awesome.”