Daniel Stenberg's HTTP client and library. Shipped in every operating system; the single most ubiquitous program after maybe sh.
From Wikipedia
cURL is a free and open-source computer program for transferring data to and from Internet servers. It can download resources identified by URLs from a web server over HTTP and supports a variety of other network protocols, URI schemes, multiple versions of HTTP, and proxying. The project consists of a library (libcurl) and command-line tool (curl), which have been widely ported to different computing platforms. It was created by Daniel Stenberg, who is still the lead developer of the project.