There are plenty of options for powerful server side languages, but I can’t think of any strongly typed, truly powerful client side languages. Javascript does a lot with forms, basic math, and interacting with the server with AJAX and such, but it has it’s limits when compared to other languages.
I’m looking for a strongly typed client side language capable of image processing or data crunching, raw things like that. Or if there are javascript libraries that help enforce data types of variables, that might work too.
In the browser as far as strong typing goes you have Java with GWT (static), Python (dynamic) with Pyjamas, Dart, Opa (static) and a bunch of languages that can cross compile to javascript from LLVM (C, C++, Objective C, Java, Ada, and many many others) thanks to emscripten.
In terms of performance you can take a look at Google Native Client and Javascript typed arrays.
Personally I think Opa is one of the most interesting web languages today.