3 open and modern web standards for client side development. Everything running natively in the browser uses it... but not only in the browser!!
Thanks to
phonegap, you can run a html5/css3/js app on your smartphone.
With
titanium, you can generate windows/linux/macos installable apps from the same html5/css3/js code.
NodeJS project brings js to the server side, so you can reuse your js skills.