Steam's Portal 2 Workshop is filled with unique twists on the space-bending puzzler's central mechanic. With such creativity lurking in the primordial soup of the Workshop, it would take something special to crawl out and into the main Steam storefront.
That something is Thinking With Time Machine, and it fits the bill for two reasons. Firstly, it introduces a new time recording mechanic, in which you can replay your actions to create a temporary clone in-level. Secondly, it lets you look at your legs.
I've only made it through the first tutorial section so far, but already it looks to dramatically expand the complexity of the original game. Despite some wonky sections, the time machine device is an excellent creation—giving a full picture-in-picture replay of the actions that you've recorded.
Over the weekend, plenty of bugs were reported, but it seems that today's patch has fixed many of them. If not, you can find workarounds to the most common issues with this forum thread.
You can grab Thinking With Time Machine from Steam. While the mod is free, you'll still need Portal 2 to play it.