OpenSSH is a free version of the SSH protocol suite for Unix and Unix-like systems. Its current version supports SSHv1, SSHv2, Secure copy, and Secure FTP. Not only does OpenSSH provide SSH clients, it also includes the server-side software. OpenSSH does not currently support Windows-based systems.
OpenSSH initially started out as an SSH suite for the OpenBSD project. People quickly noticed that this powerful suite was secure and, most importantly, free. This lead to the eventual porting across the various Unix flavors. In fact, many Unix projects today ship OpenSSH as part of their base system. We used the OpenSSH suite extensively while writing this book.
The official OpenSSH web site is http://www.openssh.com/. This site contains documentation and other useful information about the suite. You can download the suite of tools via FTP from http://www.openssh.com/portable.html.