Several years ago, X10 offered their Firecracker starter kit for $5.90 shipped. I bought one to play around with it, and after the initial 10 minutes of fun wore off, it sat unused for a long time. However, when I was building my cabinet a few years ago, I wanted to be able to turn the arcade monitor on via software once the correct video mode was achieved. The X10 starter kit turned out to be the perfect solution for this. It comes with a serial port device (with pass-thru if you still need something connected to the serial port) which lets you control the X10 devices via software (I use a freeware DOS app in a batch file for mine). I then connected the monitor to the X10 device by taking the cord that goes from the isolation transformer to AC, and putting a standard plug on it and plugging it into the X10 Transceiver. There is a lamp module included as well, which you can use to turn the speakers or something else on and off (just don't use the dim function on speakers).

The kit also comes with a remote that you can use to manually control the devices, if you feel the need to do that.
Anyway, they're offering the same kit again for the same price of $5.90. You can get it from
http://www.x10.com/products2/cm17exitkg1b_site.htm. Just be sure to use a throw-away or spam-collector email, as they are notorious for sending out daily advertisements. You can find other software to control it, as the only software they give you for free is a remote control software that works just like the physical remote, with no scheduling of events or batch job support. There are plenty of free software solutions for DOS, Linux, and Windows if you need them. Just search for X10 Firecracker or CM17A and you should be able to find what you need.
Edit by moderator: Edited URL, no referral URLs, please!