Most websites these days feature at least one 'slider'. You may know it by another name such as a 'slide show' or 'a web banner'.
Sliders are so prevalent mainly because they are a great way to add a 'call to action' to a website
My favourite slider at the moment is Themepunch's Slider Revolution, the possibilities are unlimited and I love doing animation!
Sometimes though a custom made slider is required and I can build them too of course :)