commandlinefu.com is the place to record those command-line gems that you return to again and again.
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Streams youtube video with v=ID directly into the mplayer.
If exists, it uses the HD-quality stream.
If you don't want to watch it in HD-quality, you can use the shorter form:
ID=52DnUo6wJto; mplayer -fs $(echo "http://youtube.com/get_video.php?&video_id=$ID$(wget -qO - 'http://youtube.com/watch?v='$ID | perl -ne 'print $1."&asv=" if /^.*(&t=.*?)&.*$/')")