Microsoft Research has released a new Excel 2007 add-in for rendering network visualizations
NodeXL is a powerful and easy-to-use interactive network visualisation and analysis tool that leverages the widely available MS Excel application as the platform for representing generic graph data, performing advanced network analysis and visual exploration of networks. The tool supports multiple social network data providers that import graph data (nodes and edge lists) into the Excel spreadsheet.
The graph visualizations seem stunning for Excel. An example is shown below from Visual Business Intelligence, where the graph depicts shared Board memberships of major US companies.
More information on using NodeXL and the external people Microsoft collaborated with to create the tool can be found here at CodePlex.