DeadLine is an useful program, especially as an educative support, offering visual indications about the number of roots of an equation and a very precise estimation of those roots. It is a freeware math calculation application that's well-designed.
Here are some screenshots to illustrate the features and user interface of this application:
Qalculate
gnuplot
GeoGebra
GeoGebra Portable
Advanced Trigonometry Calculator