Mokup is an app for interactive UX/UI prototyping. Like Principle or Invision. Except it's cross-platform, it's faster and better.
All the hi-fi UI-prototyping apps are Mac only. Despite 50% of UI-designers still use Windows (statistics from Adobe).
Mokup is 100% cross-platform. Mac, Windows, Android, iOS, Web. Everywhere with native performance.
