Prototyping and collaboration for design teams

I've been a huge fan and advocate of using InVision on design and product teams for years now. You can drop in Sketch and Photoshop comps super quickly, and their interactive prototyping tools make it easy to stitch together a bunch of UIs for an experience that can get pretty close to a real product. Also leaving feedback on Boards is great, we collect feedback and it itemizes them into a list of change requests.