Wikibase

Wikibase is the software that runs Wikidata, but is also usable for other projects beyond that. Wikibase allows the collaborative editing of structured data on top of MediaWiki. It is optimized for graph-like, schema-poor data, not for tabular and strongly pre-structured data. If you want to give it a try, let us know - we would be curious and probably can help. Wikibase is an open source project, and everyone is welcome to join in development.

Features
Wikibase is an extension to MediaWiki, and relies on MediaWiki's features to provide all the functionalities of a wiki - but for structured data.

This includes: On top of that, Wikibase adds:
 * full histories of all previous versions of the knowledge base, including by whom the change was made
 * a web-based interface with a number of features for wiki gardening, user security and identity, etc.
 * a platform known to be scalable for millions of readers and tens of thousands of editors
 * world-class internationalization and localization into more than 200 languages
 * functionality to create and manage a knowledge base, including user-defined properties
 * mechanisms to access the knowledge base from other wikis and to update them on changes
 * a rich JavaScript-based UI
 * a data model that takes knowledge diversity and multi-lingual usage serious
 * exports of the data in a number of formats like JSON, RDF/XML, N3, and YAML

Installation
The easiest way to install Wikibase is to take a newly setup MediaWiki installation, and use Composer to install the Wikibase Repo and all its dependencies into the extensions folder of your MediaWiki install. (add an actual description)

Packages and code structure
Wikibase consists of a number of MediaWiki extensions and code libraries. The easiest way to install Wikibase is to take a newly setup MediaWiki installation, and use composer to install the Wikibase Repo and all its dependencies. In

MediaWiki extensions

 * Wikibase Repo: main extension for the repository wiki, where the data is collected and maintained (used e.g. on Wikidata)
 * Wikibase Client: extension for a wiki to access and display the data from a repository (used e.g. in Wikipedia)
 * WikibaseLib: common code for the repo and client
 * WikibaseSolr: Solr-based search functionality (currently unmaintained and undeployed)
 * Wikibase Query: enables to create and save queries, as well as the maintenance and caching of query results

Wikibase code libraries
Those are pure PHP libraries and are expected to be also usable for other applications, like bots and scripts, dealing with Wikibase and Wikidata.
 * Wikibase DataModel: base library
 * Wikibase Database: access to the database
 * Wikibase QueryEngine: engine answering queries

Generic code packages

 * Diff
 * Ask
 * DataValues
 * ValueParsers
 * ValueFormatters
 * ValueValidators
 * DataTypes
 * Serialization

Development
One simple way to get started as a Wikibase developer quickly, is to use Vagrant. (add pointer to that)

Similar projects
There are numerous other projects to support the collaborative editing of structured data.
 * Semantic MediaWiki: widely used family of MediaWiki extensions for enabling to managed structured data in a wiki together with its text.
 * Freebase: Google's project of an open knowledge graph.