Celtx
Used by 6 peopleceltx.com ↗

“I use Celtx for both screenwriting and playwriting, though I'll always start out in Docs.”
Olivia Kingsley Interview, Writer, performer, director · via Uses This 
“We use Celtx to write the scripts and then to breakdown and create schedules to prepare the shootings.”
Erika Lust Interview, Erotic film maker (Lust Cinema) · via Uses This 
“If what I'm writing needs to be in screenplay format, I use CeltX.”
Avery Edison Interview, Stand-up comedian, writer · via Uses This 
“I'm sure CeltX is lovely, but if AMC tell me they only use Final Draft, I'm not going to tell them to change their entire workflow system to suit me and open source nerds, you know?)”
Warren Ellis Interview, Writer (Transmetropolitan, Crooked Little Vein) · via Uses This