If you want to convert a flash stream into a more comfortable format, that can be saved on your local disc and be played by most software/hardware, here’s a short guide:

Save your prefered video (from i.e. YouTube, MyVideo, etc.) with KeepVid.

When FFmpeg is installed, the conversion in Linux is quite simple:
ffmpeg -i input.flv -ab 128 -ar 44100 -b 900 \
-s 640x480 output.mpg

-b = videobitrate
-ab = audiobitrate
-ar = audiosamplerate (Hz)
-s = framesize width * height