Name : Opam
Homepage : https://opam.ocaml.org/
Why : It is OCaml's package manager, and thus easiest way to install tools like Merlin or Emacs plugins to make OCaml programming easier.
Open-source : Yes
How many users will anticipate it : Can't give any number but I would say any reasonably good OCaml programmer who happens to use Solus will anticipate it.