While I was reading up on these tools and checking it out, I was pretty amazed and I really wished I had those tools way back when I was in college. I studied Multimedia Arts which covered about everything from graphics, to photography, to animation, web design and development and video and there were tons of projects with groups where we were tasked to make illustrations, paintings, programs and websites and if these tools existed back then, our lives would have been easier and team projects wouldn't be hard to organize. We used to just work with sending files around and stitching everything up or just really working all at the same time in one area (a library or coffee shop) to get things done quickly. Nothing against meeting up with everyone in person, of course. I still prefer working on projects face to face, but these tools here, like Zeplin seems to bridge that gap of working far apart with your team.
If you're looking for tools that'll help with design, check out these 7 design resources. They're pretty great!