Seagull Right Angled Finder.
For someone who spends as much time as I do lying on the floor photographing it is surprising I do not own a right angled finder. I have done in the past, but not for a digital camera so far. I do have the Zig View device which is good, but not quite the same thing. http://www.zigview.co.uk/
I have now put this right, but gone for the cheaper Seagull finder rather than the Canon one. http://www.camerachina.com/productxx.asp?id=1472
It comes in a handy little pouch and numerous adaptors for many different cameras. There are two for Canon cameras, 18mm and 22mm. The larger of which fits my 1d mk1v. Importantly there is a prism so the image is the right way around, which was not the case with some of the early ones I owned in my Olympus days and trying to follow a moving subject when the image was back to front was near impossible.
The image is also very clear. It can magnify the image to 2.5x, but I can’t imagine I will ever want to do that. There is a built in dioptric correction ring. I bought it off Ebay for £27 including postage from China, which is a bargain. A Canon one would be closer to £200. I have not used it yet, but it looks well built and I expect it will get used a lot.