I'm not aware of large standalone projects that depend solely on it. Multicore is backwards compatible with stock OCaml though and 4.12 ships with some stub Atomics (https://github.com/ocaml/ocaml/blob/4.12/stdlib/atomic.mli) which makes writing programs that can work on both easier.

We also maintain some projects on top of Multicore that are pretty interesting:

* Domainslib - channels and task queue that makes using parallelism via Domains much easier: https://github.com/ocaml-multicore/domainslib

* eioio - very much a work in progress but enables high performance direct-style IO with fibers and effects. Backs on to io_uring on Linux and is designed to also work with IOCP on Windows: https://github.com/ocaml-multicore/eioio