EasySSP
Easy Sound Space Perception is a small and lightweight audio visualization tool, which currently provides spectrometer and goniometer. The main advantage of this tool is that it can view information about up to four stereo pairs at the same time using different colors (see screenshots). And more cool features are on the way, that's for sure.
As EasySSP is based on a cross-platform JUCE library it can be built in many flavors. Native VST builds are known to work in both Linux and Windows for example.