If you happen to be a lazy bastard (like me) and you want to watch downloaded videos (like me) that are still packed in a rar archive (maybe even multipart) (like mine) you could try the following:
save the following file as something.sh

unrar p inul $1 | mplayer -
p <- output extracted files to stdout
inul <- supress error messages… we don't want to see them
$1 <— the input that is given to the script (e.g. the filename of the rar archive)
| <— a pipe
mplayer – <— tell mplayer to read from stdin

should also work with a "vlc -";