For years many folks, including me, have wondered why developers don't take advantage of the DSP processing power of our graphics cards. Not such an issue today with our multi core processors and the advantages of 64 bit memory handling. Still I found this one too unique to pass up.  I have a few impulse verbs that allow me to use my impulses, reverberate, Mix-IR2, and the bundled ones, but I think I prefer the GUI in this one for it's ease of use.
 
http://gpuimpulsereverb.de/
 
Sorry if it's been posted already

I think the reason you don't see GPU acceleration for audio FX is two fold:
 
1) Not worth the overhead to stream the data in and out of the GPU in small chunks like audio needs. GPUs are more made for large block moves.
 
2) GPUs are stream processors, designed to do processing on large sets of data that is the same. So applying the same set of processing to 32 different sets of data at the same time or the like. Not really what you do with audio.