A great piece of free software for writing scripts in the standard screenplay or two-column formats is Celtx. It used to be commercial software but they have decided to make it free software.

It has a great feature to share projects between different users online, so multiple people can work on one project. You can also use their Project Central to share your scripts, receive feedback, and get the word out about something you have made.

Check out Celtx and download the latest version.