To play flash videos like the ones downloaded from youtube, you can use VLC.
You can also use to play other formats like.
*.mpg, *.vob, *.avi, *.mp3, *.ogg
Here are some uses examples.
Install it on Arch Linux using
sudo pacman -Sy vlc
Then to view a video on your disk just enter:
Do not forget to see the man page for more options.
also you can use cvlc to avoid the control window, and just view de video in a single window, with no controls.