[amirmc]

Toolchains also matter and OPAM (the package manager) has gone from strength to strength since this post. It's the basis for the OCaml Platform which combines a number of useful tools and libs into a coherent workflow (making development much more productive).

[more_original]

Also merlin is worth mentioning explicitly. It finds and highlights errors during editing (in emacs, vim and co), does autocompletion, shows types and since recently can also write pattern matches automatically.

http://the-lambda-church.github.io/merlin/destruct.ogv

[ufo]

OPAM still doesn't run natively on Windows though :(