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:

vlc /path/to/file/file.flv

or

vlc /path/to/file/file.mp4

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.