Sequential is a open source free full screen image viewer. It was designed to be able to open archives such as RAR and ZIP and more and show the images. It is also capable of viewing PDFs. This means that instead of having to deal with maximizing an application all the time, this will basically render the image or PDF in full screen mode every time.
Useful? Well, depends on what you’re using it for, but definitely can see why someone would want to use it.
CoolOSXApps < Sequential