If you think video games only come in disc form, you’re missing out on some great fun. Downloadable games and content are an extremely underrated, and in some instances, neglected area of gaming. This week is a great example. “Leviathan,” the first story-driven single-player DLC for “Mass Effect 3” was released and it helps extend the life of a great game. Also available on PSN and XBLA this week is “Rock Band Blitz.” It’s all the fun of “Rock Band” at a fraction of the cost and no need for special instruments.