Quantum GIS (QGIS) is a user friendly, open source, cross-platform, and freely distributed GIS application. With QGIS, one can easily import multiple raster, vector, and GPS-like coordinate data for the purposes of geospatial data visualization, manipulation, and map production. For users seeking more advanced functionality, QGIS can also serve as a nice graphical interface to GRASS. For more information, read the release announcement of the current version.
As of 19-Aug-2010, the latest release is QGIS 1.5.
Current releases for a variety of operating systems can be downloaded here. Note that Ubuntu 8.10/8.04/7.10 users can install an up-to-date version of QGIS using the package manager, after first adding the following line to your software sources list (replacing intrepid with hardy or gutsy as appropriate):
deb http://ppa.launchpad.net/qgis/ubuntu intrepid main