Students can draw graphs in a coordinate system. This program is good for especially high school students. The program makes it very easy to visualize a function and paste it into another program. Students can do calculations on the functions.
Philipp Heim, Jürgen Ziegler, and Steffen Lohmann. Proceedings of the International Workshop on Interacting with Multimedia Content in the Social Semantic Web (IMC-SSW 2008), volume 417 of CEUR Workshop Proceedings, page 49--58. Aachen, (2008)