Yesterday, we announced Abstract, a new tool for design teams we've been working on for the past couple of months. I posted some tweets with more information about what exactly Abstract is, how we got to this point, and what's next.
tl;dr: Git for design files wrapped in a solid desktop app. We're in private alpha right now, onboarding more and more teams every day. If you want to try it with your team, you can sign up here.
I used Frank Chimero's beautiful mark to create the icon for our Mac app.