Need a little old-school kick to start your weekend? Then look no further than SNK's developer page on the Play Store. Over the last few months they've populated it with no less than four of their classic Metal Slug side-scrollers (Metal Slugs 1, 2, 3, and X) and the top-down space shooter Blazing Star. All are available at significant discounts: half off for the Metal Slug games, bringing them to two bucks each, and a full two-thirds off for Blazing Star, which is now just a dollar.
If you've never heard of Metal Slug... well, shame on you. It started out in the Playstation era as a modern (for the time) arcade riff on Contra, featuring copious amounts of both running and gunning paired with crushing difficulty.