This instructable explains how a cheap (20 EURO) VGA monitor splitter which is used to connect one PC to two monitor can be converted into a computer controlled-monitor switcher. The final device is controlled via the parallel port and allows to turn on both monitors independently.
I use the final device in my HTPC set-up: one PC, one 15" TFT and a SVGA projector. For watching the latest news, the 15" is fine but for movies the projector is used. But then, the 15" TFT has to be turned off. I didn't want to physically turn it off and there was no easy-to-use way to control it by the PC other than switching its power on/off. This comes especially handy, if your PC (e.g. a silent barebone) only has a single VGA out. With the monitor splitter both the projector and the TFT can be used to show the PC screen. To disable a VGA output its sync signals (HSYNC+VSYNC) are disabled.
I've got the monitor switcher from pearl.de