Not combat-armor, but a lot of motorcycle protective gear is exactly this. D3O is soft/flexible normally, but goes rigid on impact. I have plates of it in my own riding gear. I believe there's also some progressive versions that harden more or less based on the impact force.
Yeah, that D3O stuff is basically just a non-neutonian putty. I was aware of the liquid/putty/gels. I was wondering if there's anything else.
"neuton"