Recently, we released RsRelayJS, a small RxJS lib that provides 'Relay' types. These are analogous to Subject types, but without the ability to call complete() or error(). Therefore, they are stateless in the sense that they cannot enter a terminal state. I've found myself using these more than Subjects in our code, to bridge non-Rx code to Rx.
Check it out here - https://github.com/Microsoft/RxRelayJS