Here it is-- a list of the best free, open-source (in most cases) screenwriting software that you may not be familiar with...
Celtx --- free, open-source screenwriting/pre-production 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. I've been following this project for a while and it seems pretty good. 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.
Openoffice.Org and NeoOffice -- Free, open-source alternatives to Microsoft Office. Compatible with (ie, reads/writes) MS Office files as well as open document format/PDF/HTML/etc. Includes word processor, spreadsheet, presentation (Powerpoint-like) software, and more. Neooffice is for Mac OS X, openoffice.org is for Windows/Linux. Also can read Wordperfect files. I recommend 2.0 "alpha" version (very stable), with the new 2.0 beta to be free at the end of August. This upcoming beta has much more OS X-like interface.
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. First thing to do is get both computers on the same network. This can be the Internet, a subnet (like you might have at home with an Airport, Linksys, etc. router), or via a direct-connection ("Create Network..." on Macs). Once the computers are both connected, the "host" computer (if it's a Mac running 10.4.x) goes to the Apple Menu, chooses "System Preferences", then "Sharing". Under "Services", check "Apple Remote Desktop". Then click "Access Privileges" and allow remote users to "Observe" and "Control". Make sure "VNC viewers may control screen with password" is checked and supply a password. Then save all the settings. Now the OTHER Mac can use Chicken of the VNC (that's the first link above) to connect to the host computer using the password. Once it does, the host's screen will be mirrored on the other computer. If you don't have a mac, or you have an older version of Mac OS X, you will need to run a VNC server manually. They are available for OS X (), Windows, and Linux. (If you use Windows or Linux, Google "VNC" for the server/client.) Finally, be sure to turn off the VNC server when you're done as a security precaution. Even though VNC is password protected, it's still a good idea to shut it off when not in use. The new version of OS X ("Leopard") will have a desktop sharing feature built into iChat.
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 so is probably fairly mature. Don't know if I'd edit something REALLY important with it, but might be something to fool around with if you are comfortable with linux.
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.
Blender (and Blender Nation) - Blender used to be a commercial 3d modeling/animation package, but the Internet raised the $ online to buy the source code out from the company that owned it. At that point, development really took off, and many cutting-edge types of features have been added (the source code is free, so lots of programmers have been fooling with it) including fluid dynamics, inverse kinematics, hair, clothing, etc. Here is a short film where all the visuals were created with Blender. Bad story, impressive visuals, released under a creative commons license, and all source files for the animation are available. Blender works on Mac, Linux, & Windows.
Cinepaint (formerly FilmGimp) is an open-source paint program developed for and by film studios for retouching high-resolution image files. Used in quite a few movies you've heard of. A spinoff of the GIMP image processing software (www.gimp.org), Cinepaint is going to be releasing a brand new version called "Glasgow" on July 20th. Best when used in Linux.
Ubuntu-- yes it's needed for some of the above -- the best Linux distribution for non-techies. Download a (free) CD image. Burn the CD. Then boot off the CD and you're in the Linux desktop, experiencing a fully-functioning preview of Ubuntu without even touching your hard drive. Play with it, see how it works on your computer, etc. Then, if you like it, you can do an installation from the same CD. PC (x86) and mac (PPC) versions available.
So there you have it. Got any more free tools you want to plug? Just comment below!