Maltego

Maltego is a forensics and information gathering application. It is an open source application with a free and paid edition for Linux and Microsoft Windows. The free "community" edition has a nag screen and reduced functionality.

Website: http://www.paterva.com/maltego/

Free "Community" Edition: http://www.paterva.com/maltego/community-edition/

Paid "Commercial"Edition: http://www.paterva.com/maltego/download/