Remember the important angles on the unit circle, which is a circle with radius 1.  The angles are found by using the pythagorean theorem to get the sides of right triangles inscribed inside the unit circle and the trig functions sine, cosine and tangent.

0, 30, 45, 60, 90, 120, 135, 150, 180, 210, 225, 240, 270, 300, 315, 330, 360

These angle measures can be converted to radians by multiplying each by Pi/180.