Interesting list of security research based search engines. Some I have heard of, hunter is a new one for me (find all email addresses for a given domain). PublicWWW is anoather interesting one, thought its based on web code. Pipl is a new one for me to, I usually use come custom google searches. It should be noted that most of these will only give you a “sample” for free. You have to pay for full results.

https://securitytrails.com/blog/hacker-search-engines