A few months ago I ran into a Windows program called Equalizer APO (beta) while I was searching on the Internet for a Windows counterpart of the OS X based program Audio Hijack 3.
Equalizer APO is a parametric / graphic equalizer for Windows. It is implemented as an Audio Processing Object (APO) for the system effect infrastructure introduced with Windows Vista.
(APIs like ASIO or WASAPI exclusive mode can not be used)
Equalizer APO, is a free program that let you tweak real-time Windows audio.
This program works with configurable modules that can be added and routed almost endlessly.
For example the modules consist of; appliances, pre-amp, gain control, 15-band GEQ, 31-band GEQ, PEQ, all kinds of filters (all-pass, band-pass, notch, low-pass, high-pass, etc.), FIR, convolution (with impulse response), delay (also per channel), etc ..
It has also the possibility of exporting/importing TXT files, for example; PEQ, convolution, Smaart v7 and other RTA software.
Personally I find two things annoying about this program, there are no VU meters and no filters like Linkwitz-Riley, hopefully they will be add soon.
Program works surprisingly well.