The 404 page is where your visitors are taken to if the page they were looking for does not exist.
So much time is spent on designing and developing a site that this page is often overlooked, but a recent post by Renee Fleck showcases some uniquely creative designs to inspire you.
Some will argue that the 404 page is actually an incredible opportunity to point your visitors to useful places (specific products, help pages, etc), and they're right. However, depending on your vision and aesthetic, there's something to be said for leaving a lasting impression. And the 404 page is a great opportunity to do that.