The game comes with around 20 shaders, but players can make their own.
To use a shader type /lws *ShaderName*.inc
You can find a complete list of the shaders you currently have if you go to Toribash, Data, Shader.
If you have the 3.9 alpha 1 or 2 then you can find a list ingame, if you press escape then go to Setup then Shaders.
To use shaders you need to enable Shaders in the options menu.