- Scala / Scala.js (http://www.scala-js.org/) and I'm actually using this one for a non-trivial app
- Clojure / ClojureScript (https://github.com/clojure/clojurescript) with some companies using it in production
Both are using Google's Closure compiler to do tree shaking minification and are emitting source maps for the compiled code, so the size of the compiled artifacts is manageable in spite of the huge standard libraries they come with and debugging works well. Normally if you want tree shaking with Google Closure, you have to write code specific for Google Closure, however these compilers are doing that for you.
What you get in comparison with Javascript are saner languages that aren't just a prettified Javascript - completely different type system, awesome standard libraries, packages, saner semantics overall; you get a sane dependency management and build process, by means of their build tools (SBT and Leiningen respectively); you get libraries that are cross-compiled for the JVM and Javascript, so you can do code sharing between the server and the browser and you can use mostly the same tools, like IDEs. For example it's a great experience to use IntelliJ IDEA for targeting the browser with Scala - you get code completion, refactoring, the works.
And yes, the compiled size of the final artifact is a source of worry, but in practice I found to not be an issue and what's awesome is that in the long run it actually saves bandwidth for heavy apps. With Javascript many people tend to avoid using third-party libraries, because every imported library adds to the download size, however with tree shaking you're paying only for what you use. As I mentioned, being a Scala developer I picked up Scala.js and it's completely awesome, in spite of the project being young. Unfortunately these heavier alternatives to Javascript haven't picked up popularity - for some reason people preferred putting lipstick on Javascript and pretend that it's a new language.
I'm happy that compilers for C# are happening as well. My only gripe is that in true .NET fashion I'm not seeing a link to the source, which makes me think that this won't be open-source. Which is a pity.