Dave's Short Course in Trigonometry by David E. Joyce
Publisher: Clark University 2002
This website is a short introduction to Trigonometry by a professor at Clark University. It is very easy to follow, is full of animations and study problems for the reader to work through. These notes are more of an introduction and guide than a full course. You should already be familiar with algebra and geometry before learning trigonometry. From algebra, you should be comfortable with manipulating algebraic expressions and solving equations. From geometry, you should know about similar triangles, the Pythagorean theorem, and a few other things, but not a great deal.
Mathematics Elementary Algebra & Trigonometry Trigonometry