When Apple first introduced the PowerPC based Power Macintosh computers in 1994, they included an innovative product called Graphing Calculator which was made by Pacific Tech. In addition to being a great way to show off the speed of the new PPC chip, this program turned out to be a useful educational tool and helped teach basic math skills to many grade school and college students. Unfortunately, with the switch to Mac OS X in 2001, Graphing Calculator was abandoned by Apple and users were left with a hole.
Apple responded to the need for a replacement beginning with Mac OS X 10.4 (Tiger) and bundled a program they dubbed "Grapher" (originally Curvus Pro by Arizona Software). Although Grapher doesn't feel as fast as the older Graphing Calculator, it offers much of the same functionality including 2D and 3D graphing, integration and differentiation support, and built in examples. In some areas such as customizing the graph display, Grapher is even more flexible than Graphing Calculator.
Grapher is freely bundled with every copy of Tiger and can be found neatly hidden in the /Applications/Utilities/
folder. Why Apple would hide this useful piece of software (which is clearly not a Utility) in the Utilities folder is anyone's guess, and as a result of this hiding many Mac users aren't even aware that it exists. Since Grapher does indeed reside on your system, it's worth checking out.
Note: Pacific Tech finally started supporting OS X independent of Apple, and you can still buy Graphing Calculator on their website today.