Before he was the angry-at-the-world cylon Cavil, he was sassy Al Calavicci and Sam Beckett's time traveling guide, in the fantastic Quantum Leap. NBC is currently hosting the first season for your viewing pleasure. [NBC]
I think one of the single best science fiction stories ever written was the two-parter (or was it three?) where Sam jumps back to stop the JFK assasination, only to fail but save Jackie instead (who was originally killed in Sam's past), changing the past to become "our" past as know it. It was freakin awesome.
I've oft wondered in my geek-riddled mind if the new Star Trek might pull something similar, since we know it will have time travel. How awesome would that be? Nero tries to change the past but actually causes it to happen how we knew it all along (just with different actors, cause I have no desire to see the Shat in a tight-fitting shirt these days)? A brilliant way to tie it all together.