Mac only: QuickTime is a fairly elegant player built into Mac OS X, so why not use it? After installing the Perian component on your system, QuickTime will be able to play nearly any video you throw at it.
We've previously mentioned Perian back in 2006 — ah, the days before iPhones, Android and when Google's "Don't Be Evil" was still kind of a clever corporate slogan. It's worth noting again, because Perian has added lots of format support, especially around the more web-facing codecs like H.264, and fixed many a bug, according to their changelog. The format that's still unsupported in Perian is Windows Media Video (WMV), which you're probably not dealing with as often now as in 2006. Still, if that's a necessary support item, you can grab the Flip4Mac package.
Perian is a free download for Mac OS X systems only.