I've always loved a good time-travel/alternate history story and Lest Darkness Falls by L. Sprague de Camp is one of my favorites.
It is the story of American archaeologist Martin Padway who's visit to Rome is interrupted by a bolt of lightning. This somehow sends him back to 6th century Rome. He procedes to use his knowledge to introduce anachronistic technology into the ancient world and changes history.
I've always wanted to play in an RPG with a similar theme. I don't know what system could do this. I imagine many could. In the alternative settings chapter in Dogs of W*A*R, the author suggests transporting modern soldier and special ops types into the world of Barbarians of Lemuria. Not quite the same but loads of potential fun.
Has anyone played in a game like this?