This paper proposes P2PTube, a very simple yet effective set-top-box system to assist diffusion of YouTube videos. We argue that, due to the spatial and temporal nature of video requests, the simplest design already provides non marginal gains. Our trace driven evaluation shows that, with moderate cache size (100MB) and nominal upload rates (500Kbps), about half of the video requests could be served by P2PTube. Interestingly, we also see that non marginal gains are already achievable with tiny caches -- which is tied to the presence of advertisement prior that the actual video requested by the user.