Complex Numbers show up all over the place in Computer Science and Engineering as well as Scientific Computing. Examples include Fast Fourier Transforms for Signal Processing, Circuit Simulation (Complex Numbers are very common in Electrical Engineering), and Fractals which get used in Graphics and various other fields. They also show up a lot in Physics programming as Complex Numbers have some very interesting relations to vectors and trigonometry. This linkage is critical to Euler's Identity:
Complex numbers are especially useful in electronics, optics, and quantum theory for describing waves and any periodic phenomena. Fourier transforms use complex numbers and are the key to working with wavefunctions, designing filters, signal integrity in digital electronics, radio astronomy, and on and on.