Sage is a platform for many different mathematical software packages. It can perform symbolic and numerical computations, and can be used to help student learn analytic techniques for solving differential equations, and to help student learn how to model using differential equations.
Sage can be found at http://www.sagemath.org.
David Joyner's free textbook "Introductory Differential Equations using SAGE" can be found at http://wdjoyner.com/teach/DiffyQ/des-book.pdf
(The image above comes from http://wiki.sagemath.org/interact/diffeq)