This was originally an article
in Make 03
where my awesome editor Paul Spinrad
actually made a replica of my controller and did a much better job documenting it than I would. For the controller board, if you need more explanation than "get a bunch of solid-state relays, connect them up to your parallel port, and go to town," check out the Make article. It also has an awesome comic that I have gone to pains trying to convince people that it doesn't star me.
With this Instructable, I'm putting in text that didn't make the cut for a size-constrained magazine article and creating a space to explore the latest options for computer control. The Win98 machine I had previously used to drive my controller can no longer by relied upon and I haven't yet conquerred external (parallel port, serial port, USB, ...) control with WinXP or MacOSX; hopefully we can collect some great pointers and links in the comments.
The original page describing this project is here