Some wonderful animations that show the sine, cosine and tangent of angles on the unit circle.

Sine Box
http://www.ies.co.jp/math/java/trig/sinBox/sinBox.html

Cosine Box
http://www.ies.co.jp/math/java/trig/cosbox/cosbox.html

Tangent Box
http://www.ies.co.jp/math/java/trig/tanbox/tanbox.html