Where does the oil leak from?

Hi, My Camry 95, 4 cylinder developed oil leak problem recently. The oil leak makes the driveway terrible. I find it comes from the driver side engine frontend, see two photos with below links. I have check my repair manul, still not sure what is the cause. What is the wired plug? Is it the problem.

I have changed many new parts in recent years, such as starter, new tires, brakes, CV boots, brake line etc. I would keep the car if it is not expensive to repair the oil leak problem. BTW, I had done some work on my car, such as radiator, plug and oil change etc.

Thanks a lot.