Then again, a time traveller wouldn't exactly advertise his presence, knowing how messed up that would make the timeline. Sure, any effect he has will only create the future he is from, but revealling your identity in situation like that just just about the worst sort of cultural contamination possible.
I have a book with a few descriptions of theoretical time machines, unlike those we have mentioned here already. I haven't been able to bring myslef to transcribe it all yet, but I see that time is drawing near.
In the meantime, I refer Seargentbig to
this old link from the 4Dthread.