Ngrok is a secure, introspectable tunnel to localhost.
Whenever you need to develop or test a site or a service you have the problem to expose it externally, or share to some people for test. Ngrok create a secure tunnel to external, simply launching a single-file, no dependancies program, you can create, monitor and control a tunnel to a specific port. Get the link, and share with other developers for testing. The paid service offer subdomains, support and more, but the free service is good to start.
Multi platform (win/mac/linux) work out of the box