Lesswrong2 is built on top of four major open-source libraries.
Vulcan is a framework for designing social applications like forums and news aggregators. We use it to handle many facets of the LW2 functionality such as data-loading, authentication and search.
React is a user interface programming library developed by Facebook that lets us define interface elements declaratively in the form of components. We use it to define how to render and manage state for all parts of the site.
GraphQL is a query language for the Mongo datastore. Vulcan mostly deals with GraphQL for us, but occasionally we use it to define APIs for accessing and mutating our data.
Slate.js is a modern javascript framework for building text-editors.
Information about this can be found on the github:
https://github.com/Discordius/Lesswrong2