qid's highly theoretical addon for ComputerCraft, which is itself a mod for Minecraft.
The purpose of this mod is to allow a ComputerCraft computer to receive and respond to HTTP requests, allowing it to act as a real web server. It's probably not going to be very fast and won't be able to handle a lot of requests, but I consider this to be more of a technical experiment than anything truly useful.
In order to make this idea slightly more feasible, it will actually include a second component, a front-end web proxy. I haven't started work on that yet, but it will likely be a Python or Ruby on Rails application. The proxy's purpose is to handle the full HTTP protocol and transform it down into a simplified protocol that CC-WebServer will speak. The idea is to get as much of the work as I can out of the Minecraft process and into the proxy, which should help performance. I also plan to implement a couple of useful features in the proxy, such as Markdown support, so that the Lua code running in ComputerCraft doesn't have to produce full HTML.