Curl is free open source software that is used for making HTTP(S) requests. It's pre-installed on a variety of non-windows platforms and is supported on almost all operating systems, including Windows. It's particularly useful for debugging APIs, as testers and developers can use identical commands for reproducing issues without the need for screenshots or browsers or any other intermediate technology. Below is a short tutorial to cover most use-cases.
curl www.google.com #returns just the body of the HTTP request
curl -i "www.google.com" #returns the body plus the HTTP headers
curl -v "www.google.com" > /dev/null #(*nix only) HTTP headers without body