The Haxe language: https://haxe.org/

https://frama-c.com/

https://fbinfer.com/

https://mirage.io/

https://coq.inria.fr/

https://github.com/ygrek/mldonkey (Stale project but large codebase.)

https://akabe.github.io/ocaml-jupyter/

https://reasonml.github.io/

https://github.com/moby/vpnkit (Used by Docker)

One that I've found pretty helpful is comby: https://comby.dev/

It's a code-syntax aware large-scale search-and-replace tool. E.g.,

    comby -matcher .scala -review 'foo(:[x], :[y])' 'foo(:[x])'

This will search in the current directory tree for all files that contain the code pattern 'foo(x, y)' and replace it with 'foo(x)', using Scala syntax rules. It's super convenient for doing large-scale codemods. E.g. https://github.com/tinymce/rescript-webapi/pull/40

here are a few

- https://github.com/coq/coq

- https://github.com/mirage/mirage

- https://github.com/returntocorp/semgrep

- https://github.com/bcpierce00/unison

See https://v2.ocaml.org/learn/companies.html for some more leads, as lots of those companies maintain useful OSS software.

FFTW is written in C, but it used a FFT compiler written in OCaml to generate snippets of C code. [1]

[1] https://www.fftw.org/fftw3_doc/Generating-your-own-code.html

A lot of projects also started on ocaml and then later moved off of it once they had succeeded by showing the concept works and got some momentum going. People like it for exploratory compiler dev, then switch off it when their language can self host. IIRC both rust and elm started like this, certainly others as well.

The XAPI toolstack is written in OCaml: https://xenproject.org/developers/teams/xen-api/

I've recently completed bugfixing/testing on 4.14.1+no-naked-pointers, and 5.0 compatibility is not far behind (we're usually 1 or 2 compiler versions behind latest, e.g. current production releases are built using 4.13.1)

Disclaimer: I work on the XAPI project as part of my job, the project itself is >15 years old at this point.

One that I use is the unison sync tool