Zelscope is described as 'Windows software that converts your PC into a dual-trace storage oscilloscope and spectrum analyzer'. There are four alternatives to Zelscope for Windows, Linux, Mac, SAVIHost and VSTHost. The best alternative is Signalizer, which is both free and Open Source. Other great apps like Zelscope are Scope (Free Personal), Visual Analyser (Free) and xoscope (Free, Open Source).
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.