Where can you use CoffeeScript, and how? To learn about the language head to http://coffeescript.org. Then you can practice your chops at http://jsfiddle.net. I find http://js2coffee.org very nice for going back and forth between CS and JS.
When you're ready to start using CS in your app, there are a couple of options. If you're using Node.js - it's already built in! .
It seems that most Web Development environments support it in one way or another - here's a plugin for Eclipse.
Where do you stand on CoffeScript or compile-to-JS languages in general?