Celtx -- free, open-source screenwriting/preproduction software. Somewhat comparable to Final Draft. Works on Mac/Windows/Linux. Online support via a forum where you can ask the developers and other users questions, or request features, submit bug reports, etc. To transfer scripts from Final Draft into Celtx, save as "Formatted Text" and then import into Celtx. Then check to make sure it did it right. Also allows you to markup script for production breakdown, etc. See site for video demo of how it works as well as a list of new features and bugs fixed in this new version.

Script Buddy -- The Script Buddy web site provides a rather useful tool for addition to the arsenal of an aspiring writer. There are two versions of the tool: one is free, and a more robust and feature-rich subscription version.
Script Buddy is a web based screenwriting tool, which enables the user to piece together a screenplay by entering each component of the screenplay by using the icon-based interface to create a screenplay.

CineMatrix -- The Fifth Generation of CineMatrix™ is once again setting the bar for Filmmaking Software light-years ahead of anything else ever conceived of in the marketplace. This is an industry changing Revolution, a paradigm shift that changes everything you know about filmmaking software.
CineMatrix™ has done the unthinkable - we've made CineMatrix™ Industrial 5.0 available to filmmakers worldwide, regardless of economic level at No Cost. That's right, the CineMatrix™ User license that once cost several hundred U.S. dollars will soon be FREE to every filmmaker on the planet, studio and independent filmmaker alike.
 

Chicken of the VNC (Mac) -- VNC is a free program you can use to "mirror" screens with a writing partner so that you can both observe the same application simultaneously.

Cinelerra -- A Linux-based non-linear editor. Open source. I've never used it, but it's gained quite a following and has been around for a few years now and fairly mature.

Audacity -- very simple audio multitrack audio editor. It's free, and a new version is coming soon that can do a lot more than the one that's released now. But something to keep an eye on if you need very basic multitrack editing. Mac, Windows, & Linux.