For attendees of StripeCon Europe
Just tried one
You can open PSD files in the browser and measure them, extract layers etc. Not as good as an Export from Sketch (http://utom.design/measure/), but better than Photoshop (for a developer).
Make it a fun to create spec for developers and teammates! Automatically generating html page with one click, inspect all the design details including CSS Styles on it offline.
Open design files like PSD, PDF, Images. Extrating layers, measuring distance, comparing difference, commenting, revision -- All cool features for developers working with a design file.