YUILight is a customisable YUI Script that will highlight an element on your page simply by applying a class of 'yuilight' to that element. When that element is clicked and focused on, everything else on the page will be covered by a mask and that single element will stand out from the rest.
You can also configure the opacity of the mask element and the animation speed that it shows and hides. I will be adding more customisable fields later on but if you have any requests please feel free to leave them in the comments section at the bottom of this post.
I've prepared a little example page here which will guide you through the various options in a little more detail. Comments welcome below and please enjoy the script!