Well many games today have violence theme. Violence are easily learned and performed by younger audience. Thus, their action will affect the third-party. Vandalism, robbing, raping etc, are one of the common negative externalities by video games.
http://pcgmedia.com/three-teens-arrested-alleged-gta-v-robbery-stabbing/
http://www.bbc.co.uk/news/uk-england-london-21384708
The link is about teens robbing and stabbing, because of GTA 5. Many people want to ban the game because of the excess blood, violence in game......don't ban it! I haven't play yet :(...
Another negative externality by video games....health problems! Hurray!
http://www.sciencedaily.com/releases/2011/05/110517151253.htm
Science suggests that playing more video games, will results in more food intake. I find this pretty suspicious, because I play videogames, and I don't take much food...
POSITIVE EXTERNALITIES?!?!