Google's Native Client [1] is one answer. It allows you to run native code safely in the browser. It's goal is also to support LLVM IL to run safely in the browser. This way you could potentially use all your favorite languages at nearly full speed, and also have access to the DOM and other browser APIs.
Another answer is to compile your favorite language down to Javascript. This gets you the expressiveness at the cost of a slowdown in runtime speed. Languages like Coffeescript are very close to Javascript so they don't incur much overhead by doing this, but even this approach can't go as fast as code running on Native Client.
Another answer is to compile your favorite language down to Javascript. This gets you the expressiveness at the cost of a slowdown in runtime speed. Languages like Coffeescript are very close to Javascript so they don't incur much overhead by doing this, but even this approach can't go as fast as code running on Native Client.
[1] http://code.google.com/p/nativeclient/