Data analysis and visualization

http://en.wikipedia.org/wiki/Gnuplot
List of graphing software
http://en.wikipedia.org/wiki/SciDAVis 
SciDAVis is a free application for Scientific Data Analysis and Visualization.

http://scidavis.sourceforge.net/download.html
all necessary links and files for Windows can be found here.
http://sourceforge.net/projects/scidavis/?source=dlp
=========================
http://en.wikipedia.org/wiki/HippoDraw
http://www.slac.stanford.edu/grp/ek/hippodraw/
Quick links

HippoDraw canvas window

HippoDraw provides a highly interactive data analysis environment. It is written in C++ with the Qt library from Trolltech. It should run anywhere Qt runs which is just about everywhere. This release has been tested on Red Hat Enterprise Linux 3 and 4, Fedora Core 5, Microsoft Windows XP, and Mac OS X 10.4 both native and with X Windows.
Windows binaries
 The pre-built binary distribution for HippoDraw is at ftp://ftp.slac.stanford.edu/users/pfkeb/hippodraw . Included are the .dll libraries for external packages Python, Boost.python, cfitsio, ROOT, WCSlib and Minuit.

Note:

Starting with the 1.20.6 release,HippoDraw binary distribution includes libraries for Python 2.5 release; earlier versions used Python 2.4.

See Microsoft Windows for installation notes for Windows.=========================
http://en.wikipedia.org/wiki/LabPlot
http://labplot.sourceforge.net/
LabPlot is a free software data analysis and visualization application built on the KDE Platform. It integrates well into a KDE workspace but can just as well be used in another desktop environment.

3D surface plot in LabPlot 1.6

You can use LabPlot to produce publication quality plots of your data in addition to modeling mathematical functions. The latest stable release is LabPlot 1.6, based on KDE 3. However a port to KDE Platform 4 is in progress.
LabPlot allows you to import Origin data files and export plots to many different formats. LabPlot is distributed under the terms of the GNU General Public License.