HandBrake
HandBrake is a DVD ripping and conveting app. It will rip and convert any DVD and can output it in iPod or PSP format.
There is also a beta build of HandBrake that is more stripped down, and offers to only convert to PSP or iPod formats called Instant Handbrake (at the bottom of the page). I personally use this version a lot because it is hassle free and you don't have to worry about all of the daunting settings in HandBrake.
iSquint
iSquint is a free video converter. I have not come across a video format that iSquint will not convert to H.264 video formatted to the iPod. It is a must have for any owner of an iPod that can play video.
Visual Hub
Visual Hub is anoter video converter. It's a step up from iSquint, being able to convert virtually any video format to virtually almost any other video format.
VLC
VLC is the ultimate media player for any operating system including OS X, and it's free. It'll play any media file you throw at it, and it's got some great features as well. There are many video and audio filters to make what your watching all the more enjoyable.
Senuti
Senuti is iTunes spelled backwards, and it's just like iTunes, only backwards. It's a great app for getting songs off of your iPod and back on your Mac (or a friends ;).
QuickTime Codecs
I wouldn't call these apps, but these codecs are essential to improve your QuickTime experience. This codec pack contains codecs to play many types of videos such as .wmv, .divx, . xvid, and many others. There is an Intel pack, and a PPC pack. Installing these codecs is as simple as dragging them to the /Library/Quicktime folder in the Finder.
Xee
Xee is an image viewing app that can replace OS X's Preview app. It can view any image that Preview can, and then some. Such as animated Gifs.
Pixen
Pixen is an innovative graphics editor for the Mac. It's designed from top to bottom for pixel artists—people who make low-resolution raster art like the sprites you see in old video games. But it's great for artists of all arenas: Pixen is like a very powerful MSPaint or a simpler, more agile Photoshop.
Pixen allows you to create animations just as easily as you would single images. It's just drag and drop, then you've got an animated GIF, a QuickTime movie, or even a sprite sheet on your hands.
Pixen also has many other features such as editing in layers.
GIMP
The GNU Image Manipulation Program. GIMP is very close to a Photoshop alternative. However, it does not run natively in OS X, it requires X 11 to be installed, which can be found your your OS X install disk.