Thinking With Time Machine is a new and grammatically dubious Portal 2 mod that lets you record and play back time loops to solve puzzles. Considering the trouble some people had with just the portals I'm imaging a few Scanner-style head pops here.
Obviously you'll need Portal 2 to play it but the idea is intriguing, with the trailer showing past Chells standing on switches to open doors for her future self, or passing cubes across the timelines. A new datapad gadget makes it all possible, letting you stop and start recording to capture your actions and then play it back.
Thinking With Time Machine - [Steam]