Here is how to change metadata of files so you can play divx movies with itunes for example , with terminal and devtools

find /path/toyour/movies/directory -name “*.avi” -print0 | xargs -0 /developer/tools/SetFile -t “MooV”

, with applescript http://forums.ilounge.com/showthread.php?t=214705

Now that you can play those files with itunes how about having them show and play in Front Row from the confort of your remote , without even adding them to the itunes library , it is just a matter of making symbolic links in your ~/Movies folder to the directories containing your movies , you can use the shell or just alt+cmd drag them to ~/Movies to make the links.

If yo do not know by now you can download movies from youtube just by selecting the file in Safari > Activity > Copy > Paste into downloads window.

How about playing every movie format there is including the flash movies you just downloaded from youtube in anything you like , quicktime , itunes , frontrow etc , well then just download http://perian.org/ and never care about formats of movies ever again.

To top it off i have a script for those times when you do want to know what are your movie files encoded with , this is a extremely fast shell script , with basic info , you can get it packaged as a osx app to drag folders to or directly as a cross-platform script.