Many banks and hedge funds are semi famously using FP (whether that’s OCaml, Haskell, or Scala). JP Morgan has a big Haskell corner. Jane St is the company I most associate with with OCaml. Jet.com is largely built with F#.

In business domains with strict audit requirements, it’s a peace of mind thing, to know that “side effects” (aka persistence and the mutation of persisted data) are constrained to a tiny and highly testable corner of your code base.

For OSS, the examples that spring to mind for me are RabbitMQ (Erlang) and Pandoc (Haskell).