PuTTYPuTTY is a free implementation of Telnet and SSH for Win32 and Unix platforms, along with an xterm terminal emulator. An SSH server, by default, listens on the standard TCP port 22. An ssh client program is typically used for establishing connections to an sshd daemon accepting remote connections.
More Info and Downloads from here http://www.chiark.greenend.org.uk/~sgtatham/putty/
Once a user has a valid account in the server, he can SSH to the remote Linux M/C using his username and password.