VLC (initially VideoLAN Client) is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, …) as well as DVDs, VCDs, and various streaming protocols. It can also be used as a server to stream in unicast or multicast in IPv4 or IPv6 on a high-bandwidth network.
Pros: VLC supports almost every video format and has an easy-to-use interface. VLC is a cross-platform application for Mac, Windows, Linux, BeOS, Fedora Core, YOPY/Linupy, and Zaurus.
Cons: The Mac releases tend to be a bit buggy.