The main purpose for having a VU meter is obviously to monitor signal levels going to a tape recorder, reverb unit, or some other level-sensitive device, to help optimize the signal to noise ratio of the audio sent to the device. Too much level will produce distortion, and too little results in the inherent noise of the medium becoming a problem.
http://www.ethanwiner.com/meters.html
The unit described here makes no pretence at being a real VU meter, and although it can also be used as a PPM, it does not meet the original BBC standards, which call for a linear meter and a logarithmic amplifier, with highly specified ballistics.
http://sound.westhost.com/project55.htm
This project will indicate the volume level of the audio going to your speakers by lighting up LEDS. The LEDS can be any color so mix them up and really make it look good. The input of the circuit is connected to the speaker output of your audio amplifier.
http://home.maine.rr.com/randylinscott/oct98.htm
Compressor circuit based on LED and photodiode.
http://www.picks.force9.co.uk/circuits.htm