Corda
Corda is a decentralised database system in which nodes trust each other as little as possible. This reference implementation is not yet complete or offering backwards compatibility, but should be suitable for small experiments and exploration of the underlying ideas.
Pull requests are appreciated. You can chat to the team on our forums and on our Slack. It runs on the JVM and is mostly written in Kotlin.
The project is supported by R3, a financial industry consortium, which is why it contains some code for financial use cases and why the documentation focuses on finance. The goal is to use it to construct a global ledger, thus simplifying finance and reducing the overheads of banking. But it is run as a typical open source project and the basic technology of a peer to peer decentralised database may be useful for many different projects. We'd love to hear about any interesting experiments you do with it!