The best alternatives to Stack are CanvasFlip, Prototyping Tools, and BlueLibs. If these 3 options don't work for you, we've listed a few more alternatives below.
What do you think of Stack?
AutoForm — Automate the busywork from your files and your tools.
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.