Macintosh

Ripping/Converting

• Handbrake is a free utility for ripping DVDs to a format your iPod/AppleTV can read
• There is a Step-by-Step guide for using Handbrake
  
• iSquint is a free utility that will batch-convert all your media files into iPod/AppleTV format
• VisualHub is a audio/video converting application that can take almost any video format and convert it into another. This includes Apple TV, iPod, PSP, DVD, AVI, Flash and more. Screenshot
  

Codecs

• Perian is a codec pack that will help you to open almost every kind of video file.
  

Windows

• Apple TV Video Converter
  

Linux