So you want to convert you videos to watch on your Ipod, Sony Walkman, or whatever your device of choice is, than this is the utility your looking for it has many predefined encoding parameters for just those devices, it can also save in about any format you could think of or need.

From HandBrake : HandBrake is an open-source, GPL-licensed, multiplatform, multithreaded video transcoder, available for MacOS X, Linux and Windows.  Any DVD-like source: VIDEO_TS folder, DVD image or real DVD (unencrypted--protection methods including CSS are not supported internally and must be handled externally with third-party software and libraries), and some .VOB and .TS files.  Most any multimedia file it can get libavformat to read and libavcodec to decode.

File format: MP4 and MKV, Video: MPEG-4, H.264, or Theora, Audio: AAC, CoreAudio AAC (OS X Only), MP3, or Vorbis. AC-3 pass-through, DTS pass-thorugh (MKV only).  HandBrake uses a lot of (L)GPL librairies from the Linux world.  Click the blue download arrow to begin downloading HandBrake.