I am coming from an XNA background and it was really cool to re-use shaders that other people made. There was a really cool bloom shader that was available and easy to reuse. Is there an official or are there any unofficial collections of reusable shaders for water, normal mapping, or other cool effects?
The shaders in Libgdx are just OpenGL ES shaders, so any collection of OpenGL ES (or OpenGL, generally) shaders should be adaptable to Libgdx. Additionally, since WebGL is also based on OpenGL ES, WebGL shaders should be applicable.
See https://www.shadertoy.com/ or http://glsl.heroku.com/ for some "lightly organized" examples.
In practice, I've found that the inputs to the shaders need to be adapted to my specific structures, but the core of the shader can be developed in WebGL and ported to Libgdx easily.