Waveform visualisation plugin, great way to see live view on your master track and totally free. Designed for Mac/Windows Vst and AU. S(m)exoscope is one of my favourite plugins and helps you see when you have your limiter working just right. Meters like peak and RMS can lie to you, this helps you squeeze out that few extra dB and pull back on over compressed mixes.
Other free software from Bram
Well worth taking a look at the other plugins on offer. Apart from Anechoic room simulator which is just an april fools joke.