Free! MATHICS is a free, online mathematics computing engine built in the same vein as Mathematica, the best-in-class computing engine from Wolfram. The extensive reference library (all the math we can imagine) covers a number of math topics pertinent at the secondary level, and nearly every math or engineering concept covered at the university level. If you think there's no need for hyper-powered calculators in HS math, listen to this high school teacher's experience and reconsider!
