In OS X 10.3 (Panther) you can do the following.
Select the Calculator application
Using the file window or key commands Get Info
Expand the pane that says plug-ins.
Click add
Navigate to the Calculator in the Open window
It will show you the contents
Add Graphing-2D.calcview (there is also a hex, and an expression sheet .calcview file)
Launch the Calculator
Under the view menu you can select the new methods.
It doesn't have 3D and its not as fancy as you might get with mathematica but its neat and of course free!