Mac OS X has a built in VNC server. VNC is another protocol similar to Remote Desktop or Timbuktu. You can enable it through the Access Privileges part of the Apple Remote Desktop section of the Sharing preferences. After configuring this and your firewall, anyone with a VNC viewer on any platform will be able to log in.
The only downside is that whoever logs in will also have complete control of your computer. I'm not sure if there is such thing as an observe-only VNC client.
Also, a search for "share screen" on MacUpdate
shows that there are also a few third-party applications with this intended use in mind.