Javascript is everywhere. With the arrival and subsequent, frequent updates of the many libraries and frameworks built from it, Javascript has become one of the most popular languages in the world. It's hard to visit a site that doesn't make use of it in some shape or form.

Frameworks, such as jQuery, Mootools, and many more have made it very easy to utilize Javascript in a variety of ways. The key to using Javascript on a site is to realize that it is, at its core, a tool to enhance the experience of using the site, not define it. Of course, there are exceptions: maps sites, like Google Maps and the sites that utilize the Maps Javascript library, are created and designed knowing that the user must have Javascript enabled for the site to function properly for them.

A good example of this is a site I recently worked on for one of our design partners. On their site is a gallery of different types of woods, that, when clicked on, open up in a lightbox that also allows you to cycle through the images.

This is convenient because:

1. The user never leaves the page they're on.
2. The information is presented in a way that's easy to understand and operate.

Now, the key behind this is that the lightbox and cycle plugins were set up as an enhancement, not a requirement. If I were to disable Javascript and click the item again, it would instead take me to a default template page containing the same information. This way, no information is lost.

In the end, Javascript done right can make the user experience for your site more fluid and intuitive but should not be a crutch for your design to stand on in order to work properly.