You are giving a training (conference, demo) in a room with some computers and a LAN connection (Ethernet, WiFi ...). Unfortunately, the over head projector is not available (broken, stolen, whatever...). You can use a "poor man" solution.

If you have a recent Mac, activate ARD[1] on the Mac, if you have a Windows or Linux PC, install a VNC server on it. Configure it so that clients cannot take control of you computer.

On the other computers, install VNC client or use the Java viewer included in VNC server (not available in ARD). Connect all the clients to you VNC server.

This will not be comfortable or wonderful, but at least people will be able to see you slides or demo. Be careful to do things slowly and in advance on your screen because the screen refresh on the other computers will be slow.

Notes

[1] Apple Remote Desktop