QuickTime Player is to Apple as Windows Media Player is to Microsoft. It's Apple's standard video format, and if you have iTunes on your computer, you already have QuickTime Player. It works hand-in-hand with iTunes, playing back video from the iTunes Store within iTunes. But QuickTime is also a fine program on its own merit, smoothly playing back video in many formats, including the new high-definition standard H.264.
The free version I looked at can't create H.264 video, record audio for podcasts, create movies for iPods, use full-screen controls, or convert media from one format to another. The Pro version for $30 gives you all that.