I am not sure what you mean with "completely impromptu", however REF4M by Blake Vogt is a pretty cool TnR
effect which uses only one, signed card - "unfortunately" you have to rip up the card, after you have restored it.
But if presented correctly, this should not be a problem...
vibration by JM is good. It is difficult to find a good impromptu TNR since you canot actually restore a card without a gimmick. I would check out Fuze by Jeff Prace as a good alternative. It requires a gimmick, but it is one that you can make in under five seconds.
If you don't mind to carry a "small something" in your pocket to perform the TnR you can either get Torn by Daniel Garcia or Counterfeit Hollingworth by Wayne Houchin. Unfortunately Counterfeit Hollingworth has not been released yet, but if you get the chance to visit one of Wayne's lectures, you can buy the DVD there.
I believe A Taste of Chaos by Loki Kross is also impromptu. I do not own it, that is just what I have heard. By the looks of it, it is a very visual and clean TnR.
simple... you can even do it with a borrowed deck... have a spectator select a card, make a DL with another card and then use the indiferent card to tear it...then produce the original one as you like... not a good as other prepared T&R but you can work around this premise... good luck!!