NodeXL is an Excel 2007 template for viewing and analyzing network graphs, along with a set of .NET Framework 3.5 class libraries that can be used to add network graphs to custom applications. NodeXL used to be called ".NetMap".
A network graph is a series of vertices (sometimes called nodes) connected by edges. See This Wikipedia article for an overview of network graphs.
The NodeXL Tutorial by Derek Hansen, Ben Shneiderman, and Marc Smith introduces some of the core functionality of NodeXL using a few simple datasets. The current draft of the tutorial is based NodeXL Version 1.0.1.88 and was uploaded June 22, 2009. The tutorial will serve as a basis for the Communities and Technologies 2009 Workshop. It takes approximately 4 hours to go over in a classroom setting with students following along on their own machines.