OpenEthereum
The fast, light, and robust client for the Ethereum mainnet
... is to be the fastest, lightest, and most secure Ethereum client. We are developing OpenEthereum using the Rust programming language. OpenEthereum is licensed under the GPLv3 and can be used for all your Ethereum needs. By default, OpenEthereum runs a JSON-RPC HTTP server on port :8545 and a Web-Sockets server on port :8546. This is fully configurable and supports a number of APIs. You can generate documentation for OpenEthereum Rust packages that automatically opens in your web browser.