FLV - Flash video files
FLV files are video files popular on web sites like Google Video or YouTube.
You can save them via Safari's Activity window, but you can also extract them from the "Flash Video Cache" with File Juicer, as long as you don't close the browser window showing the video. FLV Files are not cached in Safari's cache but separately in "Temporary Items", which is much faster to search.
Perian
Video
To convert FLV files to an iPod friendly format I can recommend iSquint or the Perian plugin to QuickTime
Audio
If you install Perian (and restart File Juicer), File Jucier can extract the audio from FLV files as AIFF which you can drop into iTunes and turn into mp3 or aac according to your preferences by control-clicking the file.
For more general conversion try ffmpegX which handle most video file formats.