There have probably been hundreds of side scrolling shooters over the years, but one of oldest classics of the genre is R-Type, released in 1987. R-Type has recently been released on the Android Market by DotEmu.
For those of you that don't know, DotEmu is a company whose only goal is to revitalize and re-release classic retro games, keeping them alive and relevant in modern gaming culture. They've been selling games from their website for ages now, and have several iOS releases, but R-Type is their first Android port.
The game is simply about flying around in your ship and blasting away at the various extra terrestrial monsters, many of which are clearly inspired by H.R. Geiger's Alien design. The name of the game is a reference to the new class of super ship that you're driving, and yadda yadda. The game's story is just a backdrop for the game play, so moving on.
This game is just as brutal and unforgiving as ever. You fly through levels, blasting anything that moves and avoiding being hit. One single hit equals death, and you can't touch the environment at all either. As you play, you'll pick up tons of awesome weapon system upgrades which lead to dozens of possible configurations. These only last as long as you're alive though, so you really want to not die. You only start with four lives, though scoring big points refreshes them. The enemies and environments vary, and each level ends with a challenging boss fight.
Unfortunately, playing this game on a touch screen changes the experience quite a bit. You now have more control over the ship than ever, but your finger blocks part of the screen, making it easy to be blindsided by a random enemy or projectile. There are D-Pad controls, but they're far too slow to be useful. Also, If you're playing on an old phone like my Nexus One, you can't use the charge beam while moving, or your ship will end up teleporting into a wall due the glitchy screen. Newer phones should have no problem, though this aspect nearly ruined the game for me, and made a hard game even harder. Speaking of which, there is an unlockable Hard difficulty to play after beating the campaign, so have fun with that.
The retro graphics and music are fully preserved and even upgraded to be more sharp and clear. If you are old enough to get some nostalgia from this game, or you just enjoy punishing retro games, give this one a shot. It can be yours for three dollars. While you're at it, if you own an Xbox 360, you can download R-Type Dimensions, a port of this game and its sequel, packaged together with both new and retro graphics. That game is fantastic.