The best alternatives to The UI Sketchpad are KK UI Store, UI Generator, and Device Shots. If these 3 options don't work for you, we've listed a few more alternatives below.
What do you think of The UI Sketchpad?
Agree.com— Free e-signature for everyone with invoicing and payments!
Generate Lorem Ipsum UI for your design task. Populate your website, AppStore page, presentation or prototype with the unique collection of 512 fake UI templates. Play with colors, add a device frame if needed and download the result as aPNG file. 100% free.
Device Shots is a small web app that works as a tool to help you generate high-resolution device mockups with the screenshot of your website or mobile application, for free.
UiSketcher is an easy-to-use Windows application to quickly draw user interfaces with your pen-enabled windows device like the Microsoft Surface. Repetitive elements could be saved as assets and reused as often as needed.