Dealing w/ Codec Hell Under Multiple OSes?

Dixie_Flatline asks: "I just downloaded some videos from here and there, and I found myself floundering in codec hell. I've got AVIs and MPGs, and none of them play. I've got DivX and 3ivX installed, and I even tried using DivX Doctor on some of the AVIs. The other AVIs refused to be processed, and even the MPGs won't play! When I put them onto a Win2000 box, I found that only SOME of the AVIs played, most of them didn't and the MPGs were STILL broken. Is there a codec pack out there that has everything I need all packaged up neatly? How do I get myself out of this? I just want to watch my media files!" While there probably isn't a be-all-end-all codec pack for your specific OS, what web resources are useful for people who need to find a codec (even an obscure one) before they can play their media files?