All the ones I can find charge you to download the file >_>

mynoise.net/NoiseMachines/binauralBrainwaveGenerator.php

Works?

Nope, because in order to save the file, it costs.

Download <<audacity.sourceforge.net/|nya>>
Set the highlighted thing <<i.imgur.com/XISxRNb.png|nya>> to whatever your speaker output is called
Start playing the thing dunkel linked
Press record
Go get a snack, eat it, return to keyboard
Press stop
File > export > save

Also you can just make your own in Audacity but that is a little more involved and you have to know soundmath to get the frequency you want. To do that just go Generate > Tone, set the values, then repeat until you've got the frequency you want.

If you're going that route be sure to mute anything else that might be going through that audio output (Assuming you're on Windows, click the sound, then Mixer and mute system sounds plus anything else that might be making noise, apart from the browser window you want).

Generating them in Audacity is pretty easy. The math involved is: the beat frequency is the difference between the frequency in the left channel and the frequency in the right channel. So if you want 4Hz binaural beats, you could use 440Hz on the left and 444Hz on the right, or 200Hz on the left and 196Hz on the right, or whatever, as long as the difference between them is 4Hz.