I use a piece of leather with a cushion behind it...or, failing that a leather jacket with a pillow stuffed inside hanging from the wall...
Definitely an insight into the lives of fencers, not sure I wanted to have that insight!
I built a freestanding one from 2x2 screwed together with the torso covered in carpet. Next it needs an arm.
After that I'm going to add a couple of stair carpet pressure pads from Maplin connected to a Raspberry Pi via a Pi Face with some leds to indicate which pad to hit and a score readout. Should cost 60 quid or so.