VLC is the best media player in the world and is probably also the fastest. Capable of playing almost all video and audio codec VLC has become the standard to how a media player should be. The interface itself has not change making the layout simple to understand.
The are also some advance feature inside VLC which allows you to convert any codec available. As a dedicated media converter I would still recommend a dedicated video converter but if your old converting a few files VLC work great.
Streaming video is also one of VLC’s talent but take some technical knowledge to setup. If your up to setting up a dedicated media server, VLC allows you to stream multiple files using its multistream feature.
Download: VLC Mac OS X