Application/http-server for everything except static data.
Storage for unstructured, linked and timeseries data.
Storage for geo data.
Frontend test system.
Drawing of diagrams and maps.
Client Framework, integrates niceley with d3 and websockets, if you know how...
Very nice programming language, used for backend integration.
'The' editor for erlang/otp-systems, integrates niceley with git.
The one and only version control system.
The place for non-puiblic repositories. I appreciate their nice user interface and pricing model.
Webserver for static data.
I won't write pure css anymore, after getting to know this nice preprocessor.
Nice templating system, but lacks proper namespacing for non-html / xml data.
Storage for relational data
I use node.js mainly for backend-utility and data management scripts.
General communications. Almost everyone has skype, or at least knows how to use it...