Today I installed a new media player on my iBook - VideoLan Media Player for Mac OS X. It is a free software, and in fact it is GPL'ed. That's a big plus on Mac OS X, which I found many nice small software are sharewares.
One of the disappointment for me is that Mac OS X does not have a native VCD player. The built-in DVD player and QuickTimer player cannot play Video CDs, and the recommended one (MacVCD X) appears to be a commercial software, i.e. costs money. Doing some Google search reveals that VideLan client is the only note-worthy alternative.
I was actually quite surprised about the quality of VLC. Since it is a multi-platform software, I would expect that I need to open up a terminal and then run through some shell commands before I can use it. However, the Mac OS X front-end is very well done, and I have no trouble finding the right menu items to play some of the VCD's that I've borrowed from a friend. I watched Minority Report and Lilo & Stitch on it, and the quality is excellent.
VLC also plays many other formats as well (see feature list for detail), and one common use would be watching DVDs of other regions.