SSH

An SSH client gives you command-line access to a Linux server.
WindowsPuTTY
Mac: SSH is already built in. Just open a Terminal window and type "ssh" to use it.

VNC Viewer

A VNC viewer allows to access your remote desktop on a Linux server.
WindowsTightVNC VNC Viewer
WARNING: Do not install "VNC Server". The "VNC Viewer" is all you need. 
MacRealVNC Viewer for Google Chrome