YouMeScript
Recently I discovered a free screenwriting app that saves to Google Drive and also exports as PDF, FDX, or Fountain. It’s barebones — you just go to the website and start typing! Want to save your work? Install the add-on for Google Drive and it will save your files there. (You do have a Google account, right?)
So simple, so free.
As with any web app, I encourage you to download a PDF or FDX of your work after every session just in case something bad happens. Don’t trust the internet to save your work forever.