Reconnaissance/ FootPrinting/ Information Gathering

Hacking – Reconnaissance
– Active Reconnaissance

Fierce
theHarvester
SubBrute
CeWL
DieBuster
WhatWeb
Maltego

– Passive Reconnaissance

Shodan - https://www.shodan.io 
Censys - https://censys.io/ 
ZoomEye - https://www.zoomeye.org/

Wolfram Alpha - https://www.wolframalpha.com
DNSdumpster - https://dnsdumpster.com
YouGetSignal (Reverse IP Lookup) - https://www.yougetsignal.com
Pentest-Tools - https://pentest-tools.com
PIPL - https://pipl.com (Social Media)
Google Advanced Search/ Google Hacking

API REST testing tools:
Advanced REST client (Google Chrome plugin)
DHC REST client (google chrome plugin)

API Explorer:
Graph API Explorer (Facebook)
Dropbox API Explorer (Dropbox)

Archives/ Cache

https://archive.org/
http://cachedview.com/

Email Tracking Tools

Email tracker pro - http://www.emailtrackerpro.com
Didtheyreadit - didtheyreadit.com