Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.
Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Trelby - The free, multiplatform, feature-rich screenwriting program!
Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.
Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.