Fountain is a plain-text format which can be used for screen writing and rendered to a formatted screenplay.
Celtx is a fully-featured crossplatform screen-writing application. Celtx can sync with cloud services and mobile apps, although these are not open source.
Soundboard is a cross-platform tool for building and executing sound cues.
Q Light Controller is a crossplatform application for controlling DMX or analogue lighting systems like moving heads, dimmers, scanners etc.