VLC media player
VLC is a media player by the VideoLAN team designed to emphasize portability and compatibility. Historically, it was groundbreaking for Linux media software by being the first player to ship with libdvdcss, letting it play encrypted DVDs. This is now a feature that almost all players include, and is no longer an advantage.
To install, simply run
# pacman -S vlc
To install the Firefox plugin that uses VLC to play video, run
# pacman -S vlc-plugin