If you are using a modern browser, likely you already have the (arguably) most widely deployed scripting environment: JavaScript engine (or ECMAScript, if you insist). There are many things you can do with it (just look at tons of cool web apps out there). However, since it is contained in the browser, there are also things the embedded JavaScript engine can not do for you.
If you are using KDE, you also have two excellent JavaScript engines: KJS and Qt Script. You can embed either of them (i.e. KJSEmbed) and make your application scriptable. In a not-so-strange twist, they both relate (distantly) to each other via JavaScriptCore, WebKit’s default JavaScript engine, because
long time ago Apple forked KJS and used it as the base for JavaScriptCore, and Qt Script (for version 4.6 and 4.7) also uses JavaScriptCore as the back-end.
If you are interested in learning, using, and/or dissecting other open-source JavaScript engines, have a look at JavaScript Engines: How to Compile Them I wrote for our Sencha blog, which covers Mozilla’s SpiderMonkey, WebKit’s JavaScriptCore, and Google V8. The instructions should work on the supported platforms, including ARM, in case (just like me) you want to have and carry around every JavaScript engines in this planet on your Maemo-powered Nokia N900. That is fun.
As the closing, just remember, “Ask not what the JavaScript engine can do for you — ask what you can do for the JavaScript engine”.