The node.js ("core") has many useful modules.
Bookmark: http://nodejs.org/api (you will come back to it)
We are going to cover the modules labeled Stability: Stable, API Frozen and Locked. Only use the Experimental or Unstable core modules when you want to try the latest & greatest and at your own risk; avoid using them on a project for a client (unless you get agreement in writing that they want to sail close to the wind for a specific user feature).