Ever googled your own name? Yes? Then try out the following. It's amazing how much traces people leave on the internet. We need to be careful about the information, we put online. Identity theft and social engineers will abuse it otherwise.
Maltego is a program that can be used to determine the relationships and real world links between:
I challenge you to try it out! ;-)
Groups of people (social networks)
Internet infrastructure such as:
Documents and files
UPDATE (Source Security.nl): There is a launch of a similar Dutch search engine ttp://www.wieowie.nl/