From BlenderWiki

Materials Dynamic Nodes (PyNodes)

Dynamic nodes allow you to write your own custom nodes. These nodes are written in Python. See API for more specific information.

To use a PyNode load the script of wanted node in Blender's text editor. After this you need to add Dynamic node to your node setup. The node added will contain a file selector. Use it to select the file loaded in text editor. After you have loaded it, it setups the node. If the code is valid, the node will be ready to use. If not, please check Blender's console to see what is wrong.

Note that if you make any changes to the script you need to press the Update button seen on the Dynamic node.

You can find PyNode recipes at the cookbook.