When trying to play some movies on my Mac I noticed that several codecs are not part of OS X or quicktime at least. One way to fix this is to install VLC player. 

As a former user of the Microsoft platform this was my first move. I must say it works similar as on a windows based machine (that means perfect). However I forgot / did not change the filetype association for some movie files. So it happened several times that quicktime was started instead of VLC (quicktime did not play the movie because it misses the codecs). I ran into Perian “The Swiss armyknife for Quicktime” I installed it and a lot of codecs were added to the system. Now it seems that quicktime is able to play most(maybe all) of my movies!!!! Really need to have software for all movie watching Mac owners! Here is the link to the Perians Homepage.