Telnet is by far the oldest and well known remote access tool, virtually ever Unix ships with it, and even systems such as NT support it. Telnet is really only useful if you can administer the system from a command prompt (something NT isn’t so great at), which makes it perfect for Unix. Telnet is incredibly insecure, passwords and usernames as well as the session data flies around as plain text and is a favourite target for sniffers. Telnet comes with all Linux distributions. You should never ever use stock telnet to remotely administer a system.
SSL Telnet
SSL Telnet is telnet with the addition of SSL encryption which makes it much safer and far more secure. Using X.509 certificates (also referred to as personal certificates) you can easily administer remote systems. Unlike systems such as SSH, SSL Telnet is completely GNU and free for all use. You can get SSL Telnet server and client from: ftp://ftp.replay.com/.
SSH
SSH was originally free but is now under a commercial license, it does however have many features that make it worthwhile. It supports several forms of authentication (password, rhosts based, RSA keys), allows you to redirect ports, and easily configure which users are allowed to login using it. SSH is available from: ftp://ftp.replay.com/. If you are going to use it commercially, or want the latest version you should head over to: http://www.ssh.fi/.
LSH
LSH is a free implementation of the SSH protocol, LSH is GNU licensed and is starting to look like the alternative (commercially speaking) to SSH (which is not free anymore). You can download it from: http://www.net.lut.ac.uk/psst/, please note it is under development.
REXEC
REXEC is one of the older remote UNIX utilities, it allows you to execute commands on a remote system, however it is seriously flawed in that it has no real security model. Security is achieved via the use of "rhosts" files, which specify which hosts/etc may run commands, this however is prone to spoofing and other forms of exploitation. You should never ever use stock REXEC to remotely administer a system.
Slush
Slush is based on OpenSSL and supports X.509 certificates currently, which for a large organization is a much better (and saner) bet then trying to remember several dozen passwords on various servers. Slush is GPL, but not finished yet (it implements most of the required functionality to be useful, but has limits). On the other hand it is based completely in open source software making the possibilities of backdoors/etc remote. Ultimately it could replace SSH with something much nicer. You can get it from: http://violet.ibs.com.au/slush/.
NSH
NSH is a commercial product with all the bells and whistles (and I do mean all). It’s got built in support for encryption, so it’s relatively safe to use (I cannot really verify this as it isn’t open source). Ease of use is high, you cd //computername and that ‘logs’ you into that computer, you can then easily copy/modify/etc. files, run ps and get the process listing for that computer, etc. NSH also has a Perl module available, making scripting of commands pretty simple, and is ideal for administering many like systems (such as workstations). In addition to this NSH is available on multiple platforms (Linux, BSD, Irix, etc.). NSH is available from: http://www.networkshell.com/, and 30 day evaluation versions are easily downloaded.
Fsh
Fsh is stands for "Fast remote command execution" and is similar in concept to rsh/rcp. It avoids the expense of constantly creating encrypted sessions by bring up an encrypted tunnel using ssh or lsh, and running all the commands over it. You can get it from: http://www.lysator.liu.se/fsh/.
secsh
secsh (Secure Shell) provides another layer of login security, once you have logged in via ssh or SSL telnet you are prompted for another password, if you get it wrong secsh kills off the login attempt. You can get secsh at: http://www.leenux.com/scripts/.
