I was fairly gutted when the Celtx community forum closed.
Celtx used to be the best free scriptwriting software available, but more than that, it spawned a really active and engaged community of writers around the world.
It's gone a bit more up-market now, and you have to pay a subscription to use it, but I was totally over the moon to discover that there is still an old version of the software free online. It doesn't give you access to the cloud or to many of the flash new features, but it gets the job done.
There's an excellent article about it here.
You'll find the download for PC and Mac at the very bottom of it.
For me, it's not about begrudging Celtx the money - they're really great at what they do, and it's a product that's a) worth the money and b) a heck of a lot cheaper than other brands such as Final Draft.
My problem is that I'm teaching scriptwriting in an economically developing country that doesn't have access to an online payment system such as PayPal. Even if my students could afford it, they have no way of paying for it. It's a common problem in many parts of the world. Forcing them to try to format a screenplay using the teeth-grinding option of MS Word, just because of their geography, seems a little cruel.