If you're looking for screenwriting software and can't afford the $150 or more price tag of the top brands, give Page 2 Stage a try. It's free!
While I can't speak too much to how well it works, I did tinker with it a bit. I was able to flawlessly import a 120 page script that I exported from Final Draft (as RTF), and then easily edit random spots. P2S has many of the most popular features, such as multiple views (outline, index card, etc.), auto complete for character names, smart tab for paragraph style, and pagination (my favorite :D). It won't make you an award-winning screenwriter, but it will take some of the banality out of creating your masterpiece.
And if you're looking for a complete production suite including development and planning tools, you can try another free screenwriting package called Celtx.
Hell, try 'em both.