Alien Syndrome is a nice top-down scrolling arcade shooter.
The plot consists of the usual "aliens have conquered earth and you need to kick their butt". You can choose between a female and a male character, and you can play single player or dual player. Your start with a rather basic weapon, and start collecting power-ups and new weapons as you advance through the level, and killing aliens on the way. You will get points for rescuing humans who are scattered along the level, and save them from some kind of alien glue which keeps them in captivity. Each level has a mega alien boss.
Graphics are rather nice, considering this title was released in 1989. Controls are ok, using arrow keys to move and CTRL key to shoot, and also you can choose playing using your joystick.