I am not sure how many times you've gone through this scenario, but I've often tried to find someone's email address online, with no luck. This has happened mostly while visiting a company's website and looking for x person or department's contact information.
Ideally you'd expect a decent Contact Us page on that website, but many-a-times there isn't one because people forget, or because they don't want to share their personal information. Honestly, I've even found cases where an email address is so hidden that it becomes almost impossible to find.
Hunter is an online platform that lets you look for almost anyone's email address... in seconds. This doesn't necessarily work with specific pages (i.e. LinkedIn, where email addresses are often hidden from you on purpose), but it works brilliantly if you're trying to reach someone who works at a certain company, or even if you're looking for HR emails while in your job application process!
Hunter lets you find the email address of a specific person, or a company, and even verify email addresses. Here's a quick example. Say I wanted to get in touch with the founder of a very popular tech blog (iflsmartgadgets.com) but wasn't sure how to contact them. I'd simply enter the website in Hunter's search bar, and voila.