I used to use the Combined Community Codec Pack (CCCP) as I could play virtually all media with those codecs. However, after installing VLC (www.videolan.org) on a number of machines I've never had to download a codec pack.
For Realmedia and Quicktime formats I used Real Media Alternative and Quicktime Alternative which are based on the old Windows Media Player software and work extremely well, without the bloated iTunes and Realplayer software.