JavaScript has undergone something of a renaissance recently and is now the cornerpin of most web applications. Formally known as something that was riddled with browser inconsistencies, hard to work with and problematic, it is now better understood, better supported (by web browsers) and is now viewed in the same way as cascading style sheets - as a way of progressively enhancing a website.

highly visible JavaScript is unobtrusive and is used to passively enhance the user experience. A site will not cease to function when JavaScript isn't available. This Wikipedia article explains in more detail the principles of unobtrusive JavaScript.

I frequently use the JQuery Javascript library which speeds up Javascript and Ajax development by factoring out many of the browser inconsistencies that still exist.