3D Calculus Plotter (website, Java-based): A set of plotting tools that are tailored to fit the Vector Calculus curriculum
Virtual TI calculator (freeware, Windows only): To use, first extract the zip file. Then double click on Vti (or Vti.exe) to launch the program.
Winplot (freeware, Windows only): A powerful graphing package developed by Rick Parris, a math instructor at Phillips Exeter Academy in New Hampshire
Graphing Calculator: A powerful and easy-to-use graphing and algebra software package developed for the Mac and also available for Windows machines ($40 student price).