The Screenwriting Group Forum

Final Draft and Movie Magic Screenwriter are the industry standard programs, and each one retails for around $250. I own and have used both of them extensively. There are a lot of features in these programs and, while there's something to be said for using industry-standard programs as a foundation, they're really expensive compared to the alternatives. If you're just exploring screenwriting for the first time, I'd only recommend you start with one of these programs if you've got the extra money to spend or are relatively sure that you're going to take screenwriting seriously enough to make the investment pay off. Both programs have free demo versions available:
         Final Draft   Demo available at: http://trial.finaldraft.com/
         Movie Magic Screenwriter   Demo available at http://www.screenplay.com/t-mmsdemo.aspx

Fade In Pro is a program developed by a guy I know who wanted a cheaper screenwriting program option that didn't sacrifice functionality. I own and have just started using it. The full version of Fade In is only $50, making it a much more affordable option for writers who aren't quite sure if screenwriting is for them. Fade In offers a free, fully-functioning demo (the only differences between the demo and the full paid version are that the demo prints a watermark on all printed pages, and will pop up with a registration reminder after you write 10 pages). I've just started working with this program and I'm very impressed not just with it's features and customer support, but with its integration into other programs. You can save Fade In files, for example, as Final Draft files, making a script you wrote in Fade In nearly indistinguishable from one written on the more expensive "industry standard" program. This is the option I would recommend. The trial version should be enough to practice any of our article challenges, and upgrading the the full version (if you're so inclined) won't break the bank.
         Fade In Pro   Demo available at: http://www.fadeinpro.com/page.pl?content=download

Trelby and Celtx. Each of these screenwriting programs is 100% free. By all accounts, they're pretty decent basic screenwriting programs and you definitely can't beat the price. I have briefly experimented with Celtx and have not tried Trelby, so I can't offer a strong recommendation or preference for either program simply based on the fact that I haven't tested their ability to integrate with other programs, their suite of advanced features, etc. But by all accounts, if you want a full, free screenwriting program, either of these should suffice.
         Trelby   full version available at: http://www.trelby.org/download
         Celtx   https://www.celtx.com/services.html

Adobe Story is a new entry into the screenwriting software field. It's part of their Creative Cloud of products and my understanding is that they're trying to integrate it with some of their other film production programs so there are a lot of robust features they're trying to implement. I don't have any experience with this program, though, and I believe it's on a subscription basis, so those two factors combined with the fact that it's by no means a common industry program means I can't recommend it at this point.
         Adobe Story