* Originally, Google Protocol Buffers. These are still in wide use; unfortunately, they never published/blessed an official RPC stack.
* Apache Thrift, aka Facebook's answer to Protocol Buffers.
* Capn Proto, by one of the Protocol Buffers authors, now has an RPC standard!
After having used Protocol Buffers extensively, I could never go back to untyped APIs. I'd at least use Apache Thrift for everything. Hopefully Capn Proto gets more languages supported for its RPC soon.
I'm still uncertain about melding the message format and RPC mechanics though. Part of me feels like network communications is just too big of a deal to abstract away into something that looks like a normal procedure call. But hey, even if that's always true for big systems, it probably won't be for smaller apps.