At one end of the spectrum: ZeroMQ[1]. No broker to run, and everything can stay within your local network. Just add the library to your microservices and expose a port. It gives you a socket interface with well-engineered queuing primitives. It's enough for most side projects in its simplest form, while also giving you the flexibility to implement a huge number of distributed queuing patterns.

At the other end of the spectrum: just use a managed queue from your preferred big cloud provider. All are reliable enough for this use case, and the cost for most projects would be zero or negligible.

[1]: https://zeromq.org/