An inexpensive, open-source alternative to the traditional Calculus textbook (apexcalculus.com)

Other interesting open-source calculus texts:

(1) Corral, M. (super-readable!)

http://www.mecmath.net/calculus/index.html http://www.mecmath.net/index.html

(2) Feldman et al.

http://www.math.ubc.ca/~CLP/