MPlayerX (free) is one of those fabulous utilities like VLC, NicePlayer and MPEG Streamclip that seems to have ended up on my Mac as a must-have playback app. It offers an OS X wrapper around FFmpeg and MPlayer libraries. These enable it to handle a wide range of media codecs without requiring you to install extra packages.
The player provides tons of options aimed at the power user. I must be honest, though, and say that 99.999 percent of the time, I play a file, watch it and that's that.
If you want more features, the app lets you tweak playback speeds, video aspects, create loops and more. Say, for example, you are watching an out-of sync video, whose audio needs a tiny delay tweak. MPlayerX makes it simple to apply that audio delay directly in the playback window.
MPlayerX is tremendously hacker friendly. According to the developer, it integrates with third-party remote functions, offers audio pass-through for home theater compatibility and even supports YouTube video streaming.
If you wish to contribute to the MPlayerX project, there's a donate button on the site's main page.