A differentiator will take a triangle wave and turn it into a square wave, an integrator does the opposite, it takes a square wave as input and turns it into a triangle wave at the output.
However for some reason EveryCircuit seems to have trouble simulating the integrator properly, if the output is taken at the out terminal of the Op-Amp, the circuit acts as a voltage multiplier and the capacitor charges linearly but does not discharge back so it keeps charging at intervals until it reaches it's full charge.
The only way it seems to work is by taking the output at the inverting input of the Op-Amp, it turns the square wave into a triangle one (although unamplified), yet this makes no sense since this node is supposed to be at 0V (virtual ground).
