I’ve had a custom 404 page up on my web site for several years. I did it as I thought regular 404 pages were so plain. You can see mine here. It looks quite dated, though if they keep making Toy Story movies, it won’t be too bad.
There is one site who actually embeds a video into their fancy 404 page. You can see it here. Top notch work! If only I had the skill and effort to do the same. Plus, I don’t own any guns.