(ClojureScript is also the basis for Light Table, an extraordinary new tool under development for developing web apps. LT is a radical rethink of the IDE, throwing away all the persistent menus and windows in favor of a clean work table and a remarkably interactive environment that encourages test-as-you-go programming by showing you the state of your program as you execute tests. It incorporates the Code Bubbles concept, but carries it much further. Fascinating stuff, and a real vision of the future of programming.)
Anyway, this is just scratching the surface of a very large topic. For many, many more pointers, take a look at these lists:
There are caveats here, of course. All of these depend on having a modern standards-friendly browser to run on, and IE really only got to fitting that description in IE9. The vast majority of these tools are open-source and at best community-supported, and most of them are pretty immature. It’s going to be a while before we know which of them shake out to become mature environments that you can safely use for product development. But things are moving very quickly now, so odds are that within the next year some of them will start to firm up, and some (especially Closure and CoffeeScript) are now so widespread that one has to consider them decently mature.