Yes, there are free
scriptwriting softwares out there, but these are usually demos
only. Movie Magic, Final Draft, and Sophocles let you download
their free version for trial only.
On a lighter note--the only way to have
a completely "free" software is if you are a programmer, have
extra time, and can build your own, free version. Or better yet,
maybe you have a friend who is a programmer and this friend has nothing
better to do--although, these days, programmers are in high demand.
Back on a serious note--Word processing
shouldn't be ruled out completely. If in the old days scripts were
typed with a typewriter, why not a word processor? However, you
have to be very sure the format you are typing is correct. You
will have to obtain copies of actual scripts for your guideline.
This is another way you can write your script for as close to
"free" as possible.