Best software for graphing functions?

Does anyone happen to know of a program that can handle this? I have a vague memory of something called Sketchpad or a similar tool... essentially, I’m looking for software where I can simply input a function and have it instantly plot the graph for me.

You’ve got Graphistry available online, though if you're looking for heavy-duty professional tools, you’d typically turn to something like MATLAB or Mathematica...

Then there's this one—it’s an excellent little free program, actually...