A mod for Portal 2 on PC adds "time travel" options so that players may, in effect, use past versions of themselves to complete puzzles in the game.
"Thinking With Time Machine" is the name of the mod; like the original games, players can use it to get a third-person glimpse of Chell elsewhere on the map. The difference is, she's a past version of the player, loaded into the current instance through a tablet-style interactive controller.
It lets players stop and start "recording" her actions and then "play" themselves back, meaning Chell can go stand on a switch or even hand off a companion cube. See it in action in the video above.