MathCruncher is a neat app for high school math. It contains a large selection of calculators that can do geometry, algebra, and some trigonometry.
Some of its features include:
Factoring
Finding Zeros from equation coefficients
Simplest Radical Form
Quadratic Equation
Logarithms
2x2 and 3x3 Determinants
Arithmetic and Geometric Sequences
Sine, Cosine, Tangent and their inverses