Graphmatica	Shareware. Graphmatica is the best shareware graphing program available forWindows. That's only my opinion, of course, so download it and make your own decision..
WinPlot	Freeware. A function plotting program, in both two and three dimensions. Loaded with features, including animating a graph to show effect of changing the value of a parameter. That feature alone makes it a worthy addition to your software collection.