first of all, i got this idea from http://jethomson.wordpress.com/2010/02/21/diy-usb-to-serial-cable-for-3usd/.
today, I got the last of the parts to build an arduino clone and the serial cable that is used to program it.
The cable includes ground, TX, and RX, which are the minimum needed to control it. I added another wire so it can use usb power. It then looked like:
the other end was also simple. I just soldered the wires to a female header socket and covered it in electrical tape:
next: my evil coilgun.