Teamviewer. Cross platform. Easy to use. There is even a portable, no need to install, version.
The VNC protocol allows you to see and control others desktops. Since it’s a protocol there are different clients and servers.
On of them is tighvnc http://www.tightvnc.com/ which is free and open source software.
And multiplatform as well.
You do know about the built-in remote-desktop in windows? There’s also a client for Mac.
It’s not feature rich – but good for some quick maintainance.