What is node.js based on, under the hood? Is it written from scratch, or is it based on another project?
(Also, can anybody pinpoint the web server technology that makes the http module?)
Its based on similar platforms built in other languages, for example Twisted in Python, EventMachine in Ruby or libevent in C.
Its written from scratch. You can read more about it here http://nodejs.org/about/. You can also join the nodejs developer mailing lists if you want a slightly deeper answer.
Specifically, about the node.js http server, extracted from Node about page linked above:
HTTP is a first class protocol in Node. Node's HTTP library has grown
out of the author's experiences developing and working with web
servers. For example, streaming data through most web frameworks is
impossible. Node attempts to correct these problems in its HTTP parser
and API. Coupled with Node's purely evented infrastructure, it makes a
good foundation for web libraries or frameworks.
This recipe can be found in it's original form on Stack Over Flow.