Celtx is an online script writing software that comes with a free plan if all you want to do is write screenplays. This often makes celtx the app of choice for beginning screenwriters if they don't want to spend money on "bigger" screenwriting apps yet. For this it's often considered an alternative to Final Draft, although the free celtx plan and Final Draft differ greatly in terms of features. Celtx offers professional script formatting and export to pdf. See below reviews and tutorials on how to use Celtx.