SMPlayer is an incredibly flexible free media player that can handle pretty much any audio and video file format without the need to mess about downloading extra codec packs.
Its design is refreshingly simple, but spend a little time exploring its various options and you'll find it's packed with tools to help you get more from your media, including subtitles, a graphical equalizer, and adjustable playback speed.
SMPlayer can also play videos from YouTube. Just copy the URL and paste it into the player to start watching. Once it's playing, you can give it a different look using the software's selection of filters, jump forwards and back in small increments, and use various other tools not available in on YouTube itself.
Download here: SMPlayer