An optical time machine. And this one is going to take us all the way back to the beginning.

Of course, visiting the early history of the universe is interesting, but what many of us want to do is visit this planet's past. As the linked article points out:

If we want to finally work out who really killed Kennedy, all we need to do is nip over (assuming instantaneous matter transportation) to a planet some 46 light years from here and we should be able to inspect the grassy knoll (with an Extremely Large Telescope) at our leisure. Similarly, the Battle of Waterloo, the birth of Christ or the building of the pyramids — we just need to go the extra few million miles.

The real problem is getting out ahead of the light. We would need FTL drives to make that happen.

But then again, sometimes the light bounces back.