Maltego

Maltego is an open-source tool for information gather, built-in to Kali Linux. This tool grabs all the information on a specified object/person of interest. With Maltego you can search everything the Internet knows about a domain, find publicly available Emails, subdomains, etc. You can look up DNS servers, NS,MX records and a plethora of other stuff and it draws you a really nice diagram of all of these things it finds. You can interact with each component. Here is a brief overview video on Maltego:

https://www.youtube.com/watch?v=wx4mEQZM_0s