Wave is probably extremely connection and packet-heavy, so likely something evented, continuation-based or otherwise able to handle a large number of connections on little memory. Erlang, Twisted, Go, stackless, Racket WS, Seaside, Tir, ... potentially behind some sort of load balancer for the evented ones (and the non-multithreaded continuation-based ones) or they're going to block other requests.
I would probably implement business logic in Lua and do routing/messaging/connection handling with Erlang.
Stackless might work, it would be great for a prototype. Too bad there isn't a Python on Lua or a Python to Lua translator. The Lua runtime is soo much better and less resource intensive.
