But I certainly do not believe that Scala.js has a reasonable chance of having an impact alike GWT or such; maybe it can get close to (ruby) Opal or consorts, though. The main selling point of Scala still is Akka, and, at least in the web-dev world, the Play framework, I guess.
I am not using Akka, I am not using Play, I only use Scala because it is currently the language I can express my thoughts in the most elegant way, together with its industrial strength ecco system AND BECAUSE IT COMPILES TO JAVASCRIPT, TOO!!!
GWT never convinced me, Java is less productive than Javascript, so what is the point (except for legacy Java programmers ...).
Scalatra - Scala's Sinatra
Blue Eyes - appears defunct but good example of pure asynchronous framework
Lift - strong built-in security, Netty integration wip.
Probably a couple more I'm forgetting.