Remote Access Tools
TeamViewer on Windows
TeamViewer provides an easy way to login and remote control a PC. It allows for remote assistance as well, with users granting access to a remote helper and monitoring actions. It can be used without user registration, as access codes and passwords are automatically provided.
TeamViewer on Linux
Install using these commands on Ubuntu:
sudo dpkg --add-architecture i386
dpkg -i teamviewer_linux.deb
sudo dpkg --remove-architecture i386
However, TeamViewer requires around 17 MB of extra packages on Ubuntu and may not be useful on a headless server. It also requires a framebuffer (which may not exist on a headless server) and local access to install.