tarpc - RPC framework for Rust with a focus on ease of use.
Simple GoRpc - RPC explained by writing simple RPC framework in 300 lines of pure Golang.
Facebook Thrift - Serialization and RPC framework for service communication.
NNG: Lightweight Messaging Library - Lightweight, broker-less library, offering a simple API to solve common recurring messaging problems, such as publish/subscribe, RPC-style request/reply, or service discovery.
qtalk - Minimal set of packages for several languages that achieve an opinionated but expressive way for programmers to get their programs to interact.
libqi - NAOqi core framework. Middle-ware framework that provides RPC, type-erasure, cross-language interoperability, OS abstractions.
smf - RPC system and code generation like gRPC, Cap n Proto, Apache Thrift, etc, but designed for microsecond tail latency.
Cap'n Proto - Insanely fast data interchange format and capability-based RPC system. Think JSON, except binary. (Code)
Deepr - Specification for invoking remote methods, deeply.
rpc-bench - Benchmarking various RPC implementations.