RealPlayer is a video player, native for Linux and based on the Helix engine.
To install RealPlayer, you will first need the Debian multimedia repositories enabled. Just edit file /etc/apt/sources.list as root and add:
deb http://debian-multimedia.org/ lenny main
Next, issue this command as root:
wget http://debian-multimedia.org/gpgkey.pub -O - | apt-key add - && apt-get install debian-multimedia-keyring
Then update the list of packages and install RealPlayer:
apt-get update && apt-get install realplayer
That should do it.