.NetMap start a series of announcements by our group today. Marc Smith and his team have done a great job at delivering a network visualization plugin for Excel; and it’s open source.

NetMap is a pair of applications for viewing network graphs, along with a set of .NET Framework 2.0 class libraries that can be used to add network graphs to custom applications.
A network graph is a series of vertices (sometimes called nodes) connected by edges. See this Wikipedia article for an overview of network graphs.
.NetMap was created by Marc Smith's team at Microsoft Research.