SAGE is a free open-source mathematics software system which combines the power of many existing open-source packages into a common Python-based interface.

You can download this software or use it online at the following address
To try sage online you can do so by following the appropriate links at the above address and selecting one of the OpenID providers (say, for example, Google or Yahoo). SAGE is easy to use to plot direction (slope) fields of a differential equation using the command plot_slope_field. For example, the snapshot below shows the SAGE commands to plot the direction (slope) field of the differential equation
dy/dx = sin(x)*sin(y)

Alternatively, if you have access to the software MAPLE, the direction (slope) fields of a some autonomous differential equations (and the relative commands that produce them) can be found here.