EDIT : This tool is available from OPAM, please remove it. My bad.

Name : ocamlbuild
Homepage : http://ocaml.org/learn/tutorials/ocamlbuild/ or https://github.com/ocaml/ocamlbuild

Why : There's no support for OCaml in CMake so this is by far the best build tool we have

Open-source : Yes

How many users will anticipate it : Same as OPAM, any dev who needs a solid build tool for OCaml projects

Source : https://github.com/ocaml/ocamlbuild/archive/0.11.0.tar.gz