VLC is one of the first steps I recommend when somebody tells me they have an issue with video playback. Just install the small file and try opening the video in VLC. VLC by and large removes the need for external codecs through natively supporting many different file types and encodings. Supporting DivX, XviD, avi, mkv, DVDs, and many others, VLC is a powerful tool without the hassle. It also includes the ability to be installed that will allow for browser embedded objects to be caught by VLC.
There is some room for improvement, like real-time playback while moving through the clip (fast-forwarding or rewinding) and a smoother, more fine-tune volume control, but that is mostly in the interface, not the playback. If you want something to play randomly stumbled upon videos or videos with codecs you don’t want to have to randomly stumble upon, VLC is probably a good choice.