Ling, the Erlang VM directly on Xen seems neat.[0]

Folks have talked about composable OS frameworks / domain-specific OSes in the usual suspects of Erlang, OCaml, perhaps Haskell... especially useful for soft-realtime / embedded.

Irdis is neat.[1]

0: http://build.erlangonxen.org/

1: http://www.idris-lang.org/