Sketch (https://www.sketchapp.com/)
It's hard to imagine what life was like before Sketch. You'd work with a UX designer, who would pass you PNGs and PSDs. There would be designs, and then there would be a whole 'nother set of files for specifying dimensions, colors, typefaces, sizing, kerning, and a whole slew of other stuff.
You'd also have to get asset packs from your designer - for mobile devs you'd get a large set of assets of different resolutions and sizes specced for different device types.
Need a design change? Be prepared for another mega-sized download from Dropbox, not to mention the labor intensive task of the designer re-exporting all of these bits.
Then comes Sketch - good for anything from wireframing to pixel-perfect designs. All measurements can be taken directly from the app, so you send the working project file to the engineer and the engineer can take all the measurements they need. Automatically accounts for dp/px translation. Automatically exports all relevant sizes/resolutions for assets into their expected native formats. Color palettes embedded, type parameters embedded.
It's saved us mobile devs so much time.