I am loving this Firefox Extension – it not only stops YouTube videos from auto-playing (something I hate) but it also blocks the just announced Google Overlay ads (which I have luckily not experienced yet, and hopefully now never will!) 

TubeStop is an extension for Mozilla-based Web browsers that disables the autoplay on YouTube videos. This means that you can open multiple YouTube videos in tabs in the background without them all starting to play at once. TubeStop also disables the autoplay on YouTube videos embedded on sites (MySpace, for example).

TubeStop :: Now I Have a Blog Too