SSH (Secure SHell) is a network protocol that covers authentication, encryption and the integrity of data transmitted over a network. Below is a quote from Wikipedia:
Secure Shell or SSH is a network protocol that allows data to be exchanged using a secure channel between two networked devices. Used primarily on Linux and Unix based systems to access shell accounts, SSH was designed as a replacement for Telnet and other insecure remote shells, which send information, notably passwords, in plaintext, leaving them open for interception. The encryption used by SSH provides confidentiality and integrity of data over an insecure network, such as the Internet.
SSH protocol has client/server architecture. An SSH server program accepts or rejects incoming connections. This article covers the most used SSH servers for Windows.
This implementation basically wraps Cygwin's OpenSSH port in a package that includes a standard Windows installation program that provides full SSH/SCP/SFTP support. SSH terminal support provides a familiar Windows Command prompt, while retaining Unix/Cygwin-style paths for SCP and SFTP.
freeSSHd, like it's name says, is a free implementation of an SSH server. It provides strong encryption and authentication over insecure networks like Internet. Users can open remote console or even access their remote files thanks to buit-in SFTP server
KpyM Telnet/SSH Server is free, open source telnet and ssh server for Windows 2000/2003/XP/Vista. KTS provides access to the host computer via telnet or ssh protocol. It allows you to run remotely your command line applications (dir, ftp, etc.) and full colour console graphic applications (sapconsole.exe, edit.com, tetris.exe, etc.). KTS accepts connection from any telnet/ssh client running on any OS. It runs as a service and uses the native Windows identification mechanism, thus no additional user names and passwords are stored. Includes an automated install/uninstall program and a console setup.
WinSSHD is SSH server from Bitwise for all Windows NT-series operating systems. It is easy to install and configure. In a regular Windows environment, it will work immediately upon installation with no configuring. WinSSHD can be used free of charge by non-commercial personal users.
copSSH is an ssh server and client implementation for Windows systems. It is a yet another packaging of portable openssh, cygwin, some popular utilites, plus implementation of some best practices regarding security. You can use copssh for remote administration of your systems or gathering remote information in a secure way
MobaSSH allows you to run commands and transfer files on a remote Windows PC from any operating system through a fully secured and encrypted network connection. This SSH server is 100% compatible with the Linux/Unix/HPUx/AIX SSH clients, but also with Putty or WinSCP on Windows. MobaSSH Home is very simple: after downloading the installer, you can configure and start SSH server on your Windows PC in a single click!
Sysax Multi Server is a Secure FTP Server and SSH2 Secure Shell Server combined into a single product. It simultaneously supports remote access and file transfer using FTP, FTPS, SFTP, Telnet, and Secure Shell. It also supports web based file transfer using HTTP and HTTPS. The interface is very user friendly and easy to setup. This SSH server is currently used by fortune 1000 companies, educational institutions, non profit organizations, and many small businesses worldwide.