Samplers are objects that can play sounds at different pitches.
The Sampler STool lets you place samplers which can be used in a contraption to play music.
A video of the STool in action can be found here:http://www.youtube.com/watch?v=-69Fn7ZL7iY
There are options to change the sound, pitch, volume, delay, sensitivity, sampler model, visual effect.
Samplers can be activated by numpad keys, physical collision or damage.
They're tuned in semitones, so a contraption can be built to play back a tune, and other players on the server can hear it too.
The Sampler STool is packaged as a GMod10 addon, so unzip the file to your "garrysmod/addons" directory.