
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.
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.