Special angles: exact values of sin, cos, and tan, using the unit circle 

The unit circle is an underused tool for figuring out which exact values go with which special angle (30, 45, 60, 90, 120, 135, ... or pi/6, pi/4, pi/3, pi/2, 2pi/3, 3pi/4, ...). Using the definitions of cos as the x-coordinate, sine as the y-coordinate, and tan as the slope, this becomes much easier since the values of sin, cos, and tan can be estimated visually. As usual, understanding is better than brute force memorisation.
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