Jerome, when I was a young kid, I can remember using a claw hammer over the wire and crimping it between posts. That was a temporary solution at best since the crimps would eventually straighten out. I don't know of any device designed to permanently do this, but if you have enough slack to make a full 90° crimp, sometimes you can take a phillips screwdriver shaft (or something similar) and complete the crimp into a loop. Then a twist of the loop using a nail or screwdriver from time-to-time will tighten the fence. There is the danger of breaking the strand and that's no fun when that happens.
Another method is use clean cedar limbs and weave them into the fence. One or two between each post will provide some tightening of the wire strands.
Perhaps someone else will have a much better solution, but these are the two I know and I can't remember ever seeing a specific device for tightening stretched or sagging strands like you have.
