Maltego is a forensics and data mining application. It is capable of querying various public data sources and graphically depicting the relationships between entities such as people, companies, web sites, and documents. Maltego is an open source intelligence too, but isn't open source software.
For downloads and more information,
visit the Maltego homepage.
Such a cool concept and tool. The transformations and microformats make a really powerful research and visualization toolset. Start with a kernel to investigation (email, IP, domain, etc) and keep pivoting out across datasources and using new elements discovered along the way to expand outward.