Here are four movies showing rotating phasors and
how the real part of the phasor traces out a sinusoid
versus time.
Two of the movies show how rotating phasors of
different frequencies interact to produce complicated
waveforms such as beat signals.

Sine Drill (sindrill) is a program that tests the users ability to determine
basic parameters of a sinusoid.
After a plot of a sinusoid is displayed, the user must correctly
guess its amplitude, frequency, and phase.

This is an introduction to plotting sinusoids (both sine
and cosine waves) from equations.
The tutorial also reviews how to write the
equation of the sinusoid given a plot of the waveform.

ZDrill is a program that tests the users ability to calculate the result
of simple operations on complex numbers.
The program emphasizes the vectorial view of a complex number.
The following six operations are supported: