The primary editor on which to write OCaml code has always been Emacs. This tends to be a problem for users on Windows, that mostly aren't familiar with the editor. And of course there are many people that use unix platforms and don't like Emacs.
A good alternative is Eclipse, which has very good plugins for Java programming, and can be extended to handle other languages. ODT, a plugin for OCaml development in Eclipse, is being actively developed by Emmanuel Dieul. Actually there's another plugin in development by someone under the Google Summer of Code, and we hope they can integrate both efforts to get a better plugin in the end.
Anyway, ODT looks interesting right now, and it's been evolving quite fast. Eclipse is quite a memory hog, but if you already use it or are willing to pay the requirements, it can provide a modern IDE for OCaml under Windows or unix, and a good alternative for Emacs. Let's hope ODT keeps improving.