Today, I discovered how to share my Mac OS X (Leopard) screen with another non-Mac OS X computer. Not a big deal, as it is almost automatic, but I'm posting it here as I did a search and I didn't find how to do it.
What you need to do in your Mac OS X box is enabling the Screen Sharing service from the Sharing options in the System Preferences.
Before establishing the connection you need to setup a password. This is done in the "Computer Settings..." button, just like this:
Do not avoid the password step, if you do it the connection will not be established.
apt-get install xtightvncviewer
Once installed, you only need to run it and supply the IP address of your Mac OS X box and the password you configured.