Victor Agreday, Jr. over at the Download Squad -- a fellow Weblog Inc site -- has an excellent overview on the software tools that you will need in creating a film. The softwares that he does mention are either free or cheap but are professional in quality. I would add these to the list (some of these are mac only):
- Free version of Avid for your basic DV editing needs.
- Celtx (free), for all your screenplay needs.
- MPEG Streamplip (free), if you need to convert a mpeg-2 to DV. A good conversion tool with some basic editing.
- Hollywood Calculator (free) or MediaCalculator (free), if you have limited drive space and need to figure out how much room you have left, this is what you need. Also converts Film feet to Digital time-code.
- Toast, there might be a lot of burning software out there but this one is a class by itself. It is not cheap, actually it is a little expensive for my taste but well worth it.  
- Catalog or CDfinder, back all your stuff on to DVD's but cannot find the footage that you need without putting each DVD back in the machine. This is for you.

That is all I could think of, right now. Also, I am pretty sure all these Mac softwares have PC equivalents, so don't fret.