Unlike some other third party screenwriting apps, WriterDuet plays nice with screenwriting software you're already using. The app can both import and export Final Draft 8, Celtx, and Fountain files with zero formatting discrepancies, and spit out PDFs for your (or some intern's) reading pleasure. Other bells and whistles include scene-by-scene navigation, a simple set of keyboard shortcuts, and built in video chat for face time with remote collaborators. There's even a Grooveshark plug-in for those who prefer to write to their own soundtrack.
The only glitches: there appears to be no way to generate a title page or add numbers to the pages.