Mockplus is a nifty little piece of software, available for multiple platforms, that allows you to create mockups of other apps. Mostly by dragging and dropping, It lets you add icons, text, panels and other interface aspects to a template, and ultimately, to get an idea of what an app should look like before writing a single line of code.
Simple And Clean
The interface of Mockplus is a joy to use: fast, effective, and easy on the eyes, with mostly grays highlighted by oranges. There is generally enough contrast to tell the app's various icons and components apart, though some potential users with weaker vision may have difficulty. 'Icons' and 'components', by the way, are the building blocks this app will have you use, with more than 3,000 icons and 200 components respectively. Components collectively represent the various non-visual things an app can do, from buttons to scroll bars. The selection of components available is solid, but it bears noting that if your app does something specific, there may not be a component to represent that.
Before You Start Your Project
You should consider using Mockplus if the app you want to make features multiple similar screens, and doesn't have too many unique components. For most software designers or developers, at least one project they're participating in (or will participate in) matches this description. That makes Mockplus a great deal for them, and well worth trying.