This is the viewfinder that started it all: the 50mm SBOOI (1951), it's a parallax corrected viewfinder for Scewmount/Leica Thread Mount cameras. Depending on which M camera you use, and you like and use the 50mm focal length regularly, i'd recommend getting one.
Other viewfinders i'm currently using:
This is the 35mm SBLOO, also made in 1951 and parallax corrected, it's been a god-send on both my M3 and Leica III.
This is the ROSOL Finder (1933). It has parallax correction by using the small sliding frame at the rear and a swivelling peep sight for the 135, and includes frames for 50, 85, 90 and 135mm lenses. These finders are by far my favourite for ease of use and composition. They're also fantastic for low light photography.
There are a couple of different variations of this finder which i'll try and explain. The ROSOL pictured also had some siblings called RASAL and RASUK. The RASUK was the slightly fatter brother, in that he had frames for the wider following lenses: 35, 50, 73 and 90mm . You could originally buy a small mask which fitted into the RASUK called a RAMET which gave you additional frames for the 105 and 135 mm lenses.
I found a broken ROSOL a couple of years ago, and then earlier this year a broken RASUK which i was able to merge into a perfectly working RASUK. I traced the 105 and 135 frames on to cardboard. Using a scalpel, i carefully carved out the frames with a ruler, and had it laminated--Ta Dah! I now have a RASUK which covers all the frames from 35-135. This finder has been sitting on my Leica III ever since, which enabled me to retire, what i consider to be the worst Leitz finder-the VIOOH. Actually, they're pretty good, but my one was cursed. It fell off my camera once down south, and again just as i was about to get on the bus in San Francisco. For the rest of my trip, framing was a nightmare. Anyway, you can also use these framefinders on your low light M missions if required.