Ink is a plugin that helps web designers and developers provide extra information about their mockups by documenting layers, typography, effects and shape sizes, etc.
Though I’m not a web designer or developer by trade, I can totally see how the free Ink plugin would be extremely useful. Truth be told, I use Photoshop’s built-in Notes feature to document complex PSD files that I share with clients and other designers/vendors. It’s also a great way to leave notes to yourself on how you created a particular effect (the settings you used in Gaussian Blur filter, for example).
The Ink plugin is currently in beta and is available for Photoshop CS6 and CC only.