OpenSSH / PuTTY / SSH : A secure way to access remote computers
SSH (Secure Shell) is the now ubiquitous program for logging into or executing commands on a remote machine. It provides secure encrypted communications between two untrusted hosts over an insecure network, replacing the hideously insecure telnet/rlogin/rsh alternatives. Most UNIX users run the open source OpenSSH server and client. Windows users often prefer the free PuTTY client, which is also available for many mobile devices. Other Windows users prefer the nice terminal-based port of OpenSSH that comes with Cygwin. Dozens of other free and proprietary clients exist. You can explore them here or here.