Yesterday our Varian Australia software team got a USB indicator light which was quickly hooked up to our build box (CruiseControl). These lights are great. We are using cctray to play music on build pass and fails but we have found that sometimes we do miss the sound or it just gets ignored for a few hours. The light we got is programmable for a mix or read, blue, green with different intensities and flash rates. The cost was around $100.
Check them out here. Google with "usb indicator light".
It comes with a driver DLL but info to write your own is available here.
Nigel's post here.