SSH

SSH Protocol
Ubiquitous, secure protocol
Uses strong cryptography
RSA, DSA; 3DES, Blowfish, CAST128, Arcfour; HMAC-MD5, HMAC-SHA1
Interactive remote login ('ssh host')
Remote execution ('ssh host "command"')
File transfer (scp, sftp)
X11 tunneling (-X)
TCP tunneling (-L, -R)
Agent forwarding (-A)

OpenSSH
Most common implementation
Comes with most Linux distributions