You can get the biggest possible dim range with Iris.
Here is a normal screen with Brightness on 100%
If you set Iris to 50% brightness the screen will look like this
You can go as low as 10% if you enable Extended values, but this is really dark screen
Aside from going down you can also go up to 150% brightness. Here is how brightness on 110% looks like
This over the maximum brightness is really useful when watching movies for me
Looking at the other answers most programs lower the brightness by placing a transparent overlay filter.
This method reduces contrast quite a bit and may cause eye problems.
Iris and f.lux should work fine because they change the white point of the screen which is better.
One plus of Iris over f.lux is that you automate the brightness reduction based on day and night just like most blue light filter programs do with the color temperature and blue light.
Iris can do this also but this is not related to the question.
One thing to note is that I don't recommend changing the brightness via monitor buttons if the monitor is LED. Most LED monitors change the brightness via a process called Pulse-width modulation which is really bad for the eyes.
I have some explanation about PWM here:
Technically you can also lower the brightness by wearing sunglasses in front of the PC.
This will also block some UV and blue light if you don't want to use software or use some strange Linux on which no software works at all :)