GCContourPlotter

GCContourPlotter is a free Cocoa implementation of CONREC with additional code to efficiently convert to NSBezierPaths. If you have a need to plot contours in your Cocoa applicaiton, this may well be what you are looking for.
CONREC is a popular algorithm used for generating contour lines from three-dimensional height data. While more sophisticated algorithms certainly exist, for many uses CONREC is perfectly adequate, and has the advantage of being easy to understand and implement, reasonably fast, and well-known in the public domain.
Detailed information about the project and its usage can be found here.

System requirements:
- Xcode