rgb-lightning (cargo): A Bitcoin Lightning library in Rust. Does most of the hard work, without implying a specific runtime, requiring clients implement basic network logic, chain interactions and disk storage. Still missing tons of error-handling. See GitHub issues for suggested projects if you want to contribute. Don't have to bother telling you not to use this for anything serious, because you'd have to build a client around it to even try. 
Keywords: bitcoin|lightning-network