Spectroid is described as 'Real-time audio spectrum analyzer with reasonable frequency resolution across the the entire frequency spectrum'. There are eight alternatives to Spectroid for a variety of platforms, including Windows, Mac, Linux, Android and VSTHost. The best alternative is Friture, which is both free and Open Source. Other great apps like Spectroid are Signalizer (Free, Open Source), Audizr (Free), Spectralissime (Free Personal) and Visual Analyser (Free).
Signalizer is a all-in-one signal visualizing package with a bunch of unique focus-points; real-time audio visualization with optimized 3D GPU graphics, everything being scalable and zoomable gridlessly as well as being arbitrarily precise in both settings and display.
iSpectrum is an easy to use audio spectrum analyzer that allows you to view live audio in a standard frequency plot, a stereo oscilloscope view and a waterfall display. You can adjust the display resolution, center frequency and save images to disk.