Wikibase is a suite of knowledge-base software for managing linked open data, originally written for and still used by the Wikidata project.
Wikidata is the largest instance of Wikibase, a free knowledge base that anyone can edit. Its data is used by Wikipedia, by its sister projects and by anyone else who wants to make use of a large amount of open general-purpose data.
Here's our github mirror.
Before you do, you may want to review the bug report guidelines.
There's not a single canonical list—on our Showcase page we highlight several notable instances, but for more comprehensive information you can check out the voluntary Wikibase registry and the list of federated query endpoints.
Check out our federation page.
Check out our import tools page.