Unlike QuickTime, VLC has all its codecs built-in. It is the most reliable player for Mac OSX. MPlayer is very similar with similar abilities but is buggy at times so I generally recommend VLC instead.

"If" you wanted to make VLC your default player, select the video file example on the desktop, ⌘ i (command-i keys) & choose open with & scroll down until you find VLC & select. Then choose Change All, which will make VLC the default to open all files of that same file-type. You'd need to do this for all video file types you have difficulty opening, be that avi, mpeg, mpg, wmv, etc. But no, vlc won't open realplayer files. A full list of what VLC can play can be seen here http://www.videolan.org/vlc/features.html

FYI codecs for QuickTime are not always so easy to find. Here's some: 1. DivX, 2. XviD, 3. 3ivx. You can also go to QT's preferences > Update tab > Find & install new 3rd party Softw. You'd also need to go to Advanced tab > MIME settings > Select those you wish enabled.