Sorry to be "picky", but debian itself, doesn't support anything that isn't Open source/non proprietary.
That doesn't mean that you can't get it too work or find a .deb for it. Real player has been ported too linux so it's available.
At the moment, I'm using SuSE 9.3 pro, and that installed it for me by default - though you must bare in mind, that you'd often only get it "pre-configured" like that, with a "paid for" commercial version distro.
If you go to this link
you will find real player for linux. If you read the main page, you will notice that there is an "advanced installation" link for rpm based distros - unfortunately that counts you out, because debian is "apt" based - you'd need a ".deb" as opposed to ".rpm"
So if you read the install instructions it gives you a method of installng a "bin" file, it says how to make it executable then just run it (from a terminal).
Once you've done that, you should be able to make an icon on your desktop to start realplayer.
Dont forget though, there are a host of media players that you can install, probably directly from any debian mirror repositories that you have listed in your apt-sources file. For instance, my SuSE uses Kaffeine as the default video player, but theres stuff like mplayer, xine, etc etc that you could install as well - personally I've never used realplayer to play video files under linux, I just click them and see if anything is set as the default, or check out any error messages that come up.