The Youtube icon is well know and instantly communicates to website visitors that you have a related youtube channel. Now the power of youtube is not to be underestimated and if you deliver good content it can give you tenfold back in traffic and exposure. In fact the site had around 2 billion views per day in may 2010 and there a 24 hours of video uploaded every minute. The stats are breathtaking and everyone serious online should consider how to use youtube in marketing and traffic generation. In this article you will find an endless supply if nice youtube icons for your web site.
Not only youtube icons but the social section is a really good place to look.
OK if you cant find what you are looking for here I recommend that you create you own icons. It is not that hard really if you know the basic tricks.