Open Source remote control software for remote access over the internet?

I'm searching for a remote access software to be used over then internet and not from the local network. I want to use it to be able to access someone's desktop (graphical interface). I found some of them but I couldn't find any documentation on how to use them over the internet. So I'm searching for a software that does the following:

1. Is open source
2. Works on X11 and Windows
3. Can be used for remote access on the internet and has documentation on how to do it
4. It must be secure