The Canon EF 2X III Extender is a reasonably inexpensive way to double the focal length of of any L series telephoto lens. This is the very first extender I have purchased, so I can not compare it to it's predecessors. But, I am very impressed. I use it with the Canon 100-400L F4.5-5.6IS with minimal loss in quality. Much cheaper than spending $12k for an 800mm lens.
It's obviously intended for use with a tripod, as you will loose 2f stops. But I find most people won't do much shooting hand held with a 400mm lens anyway. You may be able to get away with using it handheld with some of the faster / smaller lenses. You will also have to manual focus when added to a slower lens like F5.6. If you are thinking about using this, do your homework, know it's limitations, and how to use it. It's a great addition for wildlife photography when getting closer to your subject is tricky, or not an option.