An algrebra or geometry book from high school.
The Machinery's Handbook has some general math equations in the first section of the book.
Mark's Standard Handbook for Mechanical Engineers also has curve equations in it as do FE and PE review manuals.
I know these aren't websites, but I'm more versed in my reference manuals.
As a website idea, try the math software websites: Mathematica, Matlab, Abaqus, Maple, etc.
--Scott
--Scott
For some pleasure reading, try FAQ731-376