The best alternatives to Stack are InVision, CanvasFlip, and Prototyping Tools. If these 3 options don't work for you, we've listed a few more alternatives below.
The only Visual Collaboration platform built for enterprise-wide collaboration. Freehand by InVision is exclusively designed to help all teams unlock productivity, insight, and connection.
CanvasFlip is fastest growing prototyping suite to create and validate your application design prototypes for your mobile and web applications. CanvasFlip is a cloud-based design collaboration and testing...
BlueLibs is a collection of open source products designed to create the best possible experience for starting new projects, by offering rapid prototyping tools and a set of modules to fit the most demanding projects.