Large networks visualization tool (LaNet-vi) seems to be for k-core visualization and has many nice example datasets.
Ubi Graph dynamic network visualization tool supports C, java, python, etc.
NetworkX generally useful python plug-in with many network analysis functions implemented.
Network workbench run out of Indiana Univ.
iGraph (Great tool runs on many platforms, including R. But caution, version 0.5.1, released July 2008, not all routines have yet been fully tested).
General data analysis tools:
XGobi (interactively explore high-dimensional data sets)