Run ffmpeg command with -i for input and output.avi is the output file in avi or mpg:
# ffmpeg -i input.flv output.avi
What is FFmpeg?
"FFmpeg is a collection of software libraries that can record, convert and stream digital audio and video in numerous formats. It includes libavcodec, an audio/video codec library used by several other projects, and libavformat, an audio/video container mux and demux library."