GuerrillaMail.com is very similar to the sites you mention, and the core of it appears to be this open source project:
It's not a turnkey solution, but it could be a good start toward building your own version.
Last year i found this awesome tool called Mathcha. They describe it as
Online Mathematics Editor.
a fast way to write and share mathematics
You can do quite a lot with it. In their \math container, you can write math equations in real time using LaTeX keywords. And in their \diagram container, you can draw shapes and even plot graphs. Just take a look at ...
You can use Notepad3 for that purpose. It's a very lightweigth Notepad clone that, while focused on being developer-oriented, it also has a simple and minimalist interface. Most developer features can also be disabled so you can end up with a simple plain text editor and not much more.
About your feature requests:
Preferably one that is open source