A web browser.
YouTube takes into account the device you browse the site with. The quality of the videos, depends upon the device you're using. You can force high quality vids by pasting:
at the end of the url.
Wanna share a video with someone, but you'd like to save that someone the frustration of a annoying/lengthy opening titles or introductions? Force the player to skip to a specific time in the video by pasting:
at the end of the url. Where XX = the minutes elapsed. And YY equals the seconds elapsed.