Neil has put together a useful list of free Mac apps of which some may be useful to you.
Some of my favourite freeware / open source applications for Mac, most of which have a mobile use or slant:
HandBrake: one of the best DVD rippers and encoders around, in my opinion. I've used it to put copies of all my DVDs onto my network, for streaming to other devices. £0, from HandBrake
MetaZ: tag-writing software, for audiovisual files not purchased from a store which provides tagged files. I use it to add DVD cover art, actors, synopsis and the like, before importing content to iTunes. MetaX caused me a lot of problems given the file sizes in question, whilst MetaZ has been flawless so far, although it takes a few minutes to write each tag. Sources content from Amazon as well as (usually better) crowd-sourced databases. £0, from Griff
Caffeine: stops your Mac awake, overriding default power/sleep settings. Ideal for when giving presentations (particularly when running from battery), or whenever you want to keep the screen active. £0, via Mac App Store
GPGMail: OpenPGP implementation for Mac OSX - allows signing, encryption and decryption of email: £0, from Open PGP website
OneButton FTP: this is the only FTP client I've used on my Mac, but I know there are others (CyberDuck is often recommended) - OneButton FTP has worked fine for me, though! £0, from OneButton
get_iplayer: a great program for downloading content from the BBC for time/format-shifting purposes. (I run it under Linux, but it may work on the Mac; I've never had much luck with any iPlayer-esque application on the Mac, though.) £0, from Infradead