Name:PublicWWW (Free)
URL:https://publicwww.com/
Description:Source code search engine  
Utility:This free tool helps find – 

• Alphanumeric snippet, signature or keyword in the web page HTML, JS and CSS code
• Any HTML, JavaScript, CSS and plain text in web page source code
• Sites with the same Analytics ID 
• Sites using a particular version of NGINX 
• Advertising networks users 
• Sites using the same AdSense account
• Related websites through the unique HTML code they share i.e, widgets and publishers IDs 
• Sites using certain images or badges 
• Sites that mention you
• Who is using what JS widgets on their sites 

Special Features:

• Upto 1, 000, 000 results per search request 
• API for developers
• Enables download results as CSV files
• Snippets for search results 
• Results are sorted by website popularity 
• Websites in top 1 million are revealed for free
• Web-server response HTTP header are also indexed

Alternatives are -https://www.programmableweb.com/