Griddify, "helps you compose custom grid systems, vertical rhythm, and do a bunch of other stuff with guides in photoshop."
I installed Griddify and it's great, but that's not the only thing blowing my mind. If you watch the video at the link above, look closer. It's not actually a video at all, but what seems to be an audio track synchronized to a series of clickable elements on the page.
For instance, as the creator is showing a demo of the extension, you can actually click on Photoshop panels (and input fields) in the demo. Again, it's not a video.
There's clearly a lot of Javascript trickery going on that's way over my head. Impressive.