CouchDB (used, among other things, for both the client and server of Canonical's "Ubuntu One" desktop sync service). Dynomite, SimpleDB (though reportedly some parts of SimpleDB were initially rewritten in Erlang and later ported to C++). Darcs. Mochi Media's web stack. Jane Street Capital's trading systems. Unison (popular two-way rsync written in OCaml). Flightcaster.com. Coq. FFTW. Hacker News.

I want to give a special mention to FFWT, the gold standard for Fast Fourier Transform libraries. If you use any audio or video software, you probably use this library without knowing it. An OCaml program generates the C code that compiles to the FFTW library.