I’ve published an Android app, MathGrapher, a while ago. It seems to be attracting more and more users.
With MathGrapher, you can visualize function equations, parametric equations, and/or polar equations. You can also animate an equation by defining the range of one or more parameters: a, b, c, or d.
Here are some examples:
Function equation: y = a*x^2 + b*x + c
Parametric equation: x = a*sin(t); y = b*cos(t)
Polar equation: r = a*cos(b*t + pi/2)
You can select equations from the equation gallery that contains some predefined equations.
You can save the equations for later use.