Earlier quoted context omitted.
Just about anything on the JVM (Dropwizard, Play, Finagle, Scalatra, Rest-Express, Rest-Easy, Compojure, Unfiltered, Jersey, Vert.x, Spark), Go (Gorilla, Beego, Revel), Lua (Lapis), Haskell, Erlang...
Are any of these environments actually proven in more than a handful of real-world production environments? I'd certainly like to get into alternatives but my impression was that none of these battle-tested the way Rails and Django are.
Rails is the classic example of optimizing for developer ease of use instead of performance. Which probably makes sense for startups, but scaling is a pain in the ass.
Openresty is great for infrastructure level solutions, e.g. request routing and authentication. It is used by some of the largest Internet companies in China.
The benchmarks at http://www.techempower.com/benchmarks/ are a reminder of how poorly some popular frameworks perform. And how poorly cloud performs vs relatively cheap dedicated hardware.