YouTube User Guard
UPDATE 2013/12/21 -- Google has dealt a double blow to YouTube User Guard by removing Infobars from Chrome and implementing Google+ comments in YouTube. I have not found any way to make YouTube User Guard work with Google+ comments, so YouTube User Guard is dead. The good news is it looks like Google's single sign-on now lets you switch accounts on YouTube, so you don't need this addon anyway :)
Stay signed in to YouTube forever.
Visit the Kwerty YouTube User Guard homepage for more information.