It is known to work on Linux and OpenBSD (and so will assuredly work on NetBSD and FreeBSD as well). I consider portability important, so reports of successes and failures on other operating systems is appreciated.
In short, the Firecracker/CM17a lets you turn lamps, appliances, and just about any other electrically powered device on and off with your computer. It is not the only device that lets you do this, but it is certainly the cheapest.
The Firecracker/CM17a comes with Microsoft Windows software. Flipit runs on Unix-like operating systems, so you're not out of luck if you use Linux, FreeBSD, Solaris, and the like.
Older versions can be fetched directly from the download directory.
Check out X10 Ephem for a cool program that'll let you run flipit (or any other program) at times based off of sunrise and sunset. Great for turning on lights as it gets dark, etc.
A nice page showing how an x10 can solve a real world problem, instead of silly things like turning lights on and off (which is what I use it for!): Power Cycle DSL with x10. (also some tips on getting an x10 working with Solaris)
A nifty looking CGI interface to several home automation programs is BlueLava.