Knack

I love this product. It markets itself as online database software, and surely could be used for that, but in my case I find it incredibly useful as a prototyping platform. If you have a web app concept that needs quick validation and are finding mockups and wireframes insufficient, Knack could be a huge timesaver, and a lot easier to setup and run with compared to getting the same functionality (without styling) out of Adobe products, Moqups or Sketch. A wysiwig for MVC-based web apps.