Sirens is a project mainly written in JAVA and RUBY, it's free.
A quick Arduino hack to drive a pair of "Woot-off" sirens
Just a simple driver for an Arduino-driven pair of Woot-off sirens I got for cheap.
I can put up a circuit diagram if necessary, but basically it drives a pair of relays attached to pins 4 and 5 that switch a connection to a old wall wart I sacrificed for the project.
Copy config.yml.example to config.yml, update it with your Hudson and site URLs, and run ./monitor.rb config.yml