Cluster Visualization

CLUTO
CLUTO is a software package for clustering low- and high-dimensional datasets and for analyzing the characteristics of the various clusters. CLUTO is well-suited for clustering data sets arising in many diverse application areas including information retrieval, customer purchasing transactions, web, GIS, science, and biology.
Status: not yet evaluated
http://www-users.cs.umn.edu/~karypis/cluto/index.html

gCLUTO
a graphical Cluster Toolkit based upon CLUTO
http://www-users.cs.umn.edu/~karypis/cluto/gcluto/index.html (manual)
Status: first experiments with demo dataset and twoday dataset (also see here)
Note: quite impressive little program to generate matrix visualizations and mountain views of clusterings.

METIS
METIS is a family of programs for partitioning unstructured graphs and hypergraphs and computing fill-reducing orderings of sparse matrices.
Status: not yet evaluated
http://www-users.cs.umn.edu/~karypis/metis/index.html

cviz
A visualization tool designed for analyzing high-dimensional data in large, complex data sets.
Status: evaluated
Note: CViz is a deprecated, un-impressive, non-interactive cluster tool with a restrictive user's license.see screenshot
http://www.alphaworks.ibm.com/tech/cviz