Really, any language that has threading gives you the easy concurrency that you want. Now, if you limit your choices to "concurrency, but not with OS threads",

Really, any language that has threading gives you the easy concurrency that you want. Now, if you limit your choices to "concurrency, but not with OS threads", then your pool is a lot smaller. I think Clojure (core.async), Haskell (GHC), and Rust would also give you what you're looking for.