Celtx - A complete pre-production suite...
Windows and Mac OS X, http://www.celtx.com/
From scheduling, screenplay-writing to storyboarding, Celtx covers the lot. Unfortunately some smaller features are lacking, such as the ability to print a PDF without being connected to their servers. There are also a few minor bugs, but hey, it's free!
Since pre-production is the first thing one begins with why not get straight to downloading this wonderful package.
Audacity - The free digital audio editor!
Windows, Linux and Mac OS X, http://audacity.sourceforge.net/
This free audio editing application already has a massive following, probably because it's easy to use and is packed with awesome plugins!
Even if you're a beginner or a professional audio producer, you'll find Audacity to be of great use.
Avanti with FFMPEG - Advanced video encoding
Windows, http://avanti.arrozcru.org/
FFMPEG is a handy console based conversion program. Avanti is a GUI for FFMPEG which simplifiers things a bit while still providing powerful conversion options.
In simple terms: Avanti along with FFMPEG is a powerful video encoding application. I suggest visiting the site for more details and instructions on setting it up.
MPEG Streamclip - Handy converter
Windows and Mac OS X, http://www.squared5.com/
Although not as powerful as FFMPEG/Avanti, this free video converter should be in every editor's inventory. I mean - who can have too many converters??
Although it concentrates on MPEG it does offer a few other formats and codecs (AVI, DIVX, etc)..
VLC Media Player - Compact, all round player...
Windows, Linux and Mac OS X, http://www.videolan.org/
Windows, Linux and Mac OS X, http://www.videolan.org/
Arguably the best video player in the world...
Apart from the fact that it can play almost anything, VLC media player also offers more advanced playback features such as methods of Deinterlacing and Aspect Ratios