Visualising sounds - most useful spectrum analysers?

What's the best way to see the sounds coming out of synths?

I'm trying to gain a better understanding of what is happening to the shape of a waveform when I'm twisting knobs and changing parameters. I'm wondering what programs anyone uses for live visual monitoring of sounds coming out of instruments and into the soundcard.

I've had a look at sndpeak (http://soundlab.cs.princeton.edu/software/sndpeek/) and audioXplorer (http://www.arizona-software.ch/audioxplorer/screenshots.html) but curious to hear what tools others use.