I started playing around with a computer-interfaced breathalizer after ordering an alcohol sensor from Futurlec. The sensor is very easy to use--it's just a resistive sensor that changes pretty linearly with alcohol in the air. I already had some analog interface code kicking around from making a quick 'n dirty digital computer multimeter a while back, and so with a little bit of twiddling, made a computer interface, as well as a standalone mode for my breathalizer. Some quick eagle schematiccing gave me a decent board. The natural next step, of course, was to uh...calibrate'the finished breathalizer, and so I invited some friends over to help me with the laborious process of breathing onto the sensor.
A little java graphical program will round this project off nicely. I plan to offer this as a DIY kit, and also, of course, post it on instructables. Check back soon!