Another poster child for open source software, VLC Media Player plays almost anything -- including YouTube Flash FLV files -- with no additional software, no downloads, no headaches. I use it exclusively for videos.
Unlike other media players, VLC sports simple, Spartan controls, built-in codecs for almost every file type imaginable, and a large and vocal online support community. VLC plays Internet streaming media with a click, records played media, converts between file types, and even supports individual frame screenshots.
VLC is well-known for tolerating incomplete or damaged media files. It will even start to play downloaded media before the download’s finished. It's available via Ninite.
There’s an up-and-comer called PotPlayer that’s a strong contender. Want to try an alternative to VLC? Give it a look.