HI methodman3000,
There are a lot of freeware vst oscilloscopes out there a good place to start is this page, though there are many many more
http://blog.wavosaur.com/best-free-vst-plugins-for-analyzing-audio/.
You say you want to analyse the filters output but you don't say why, i guess you want to see how the filter effects the harmonic content that passes through it?
Oscilloscopes (time domain), generally, look at the wave shape. When you filter a wave, say a sawtooth you can turn it into a sine wave and all sorts of misshaped waves, the important point here is how it effects the harmonics / timbre and to properly understand that you need a spectrum analyser (frequency domain).
I used the one that comes with Soundforge for years for spectrum analysis and have recently bought an oscilloscope, though i used the very useful
https://www.zeitnitz.eu/scope_en for a long time. This one takes a bit of routing from your DAW through a virtual cable and then you have to monitor this by turning on,, listen to cable output, in windows sound menu. Otherwise you wont hear an output. Which was okay for me as im a bit masochistic and clearly a VST or DAW based oscilloscope / spectrum analyser is a much more convenient route.
Also very good is the Yoshimasa Realtime Analyser , which is a complete package of analysis tools that was mostly aimed at room testing / setup, i believe for Hi Fi enthusiasts. It is not free and cost around $98 .
http://www.ymec.com/products/dssf3e/index.htm i think that is a very fair price for what it does though. But as above it takes a little getting used to and the routing is fairly straight forward compared to some. I use this all the time.
They can all take a bit of getting used to and can be fascinating and revealing instruments. Hope this helps a little
