The Metal Slug games are generally considered to be among the best side scrolling shoot 'em ups ever.
This online version not in English, but still well worth your time.
Originally the game was released for the Neo-Geo in 1996.
Metal Slug started receiving a larger audience when it was ported to the Sega Saturn and Sony PlayStation. It's now even available on the GBA.
Use the W,A,S,D to move around. Press J to fire, K to jump and L to throw a grenade.
Clear the area from enemy troops. Free hostages and be awarded with weapons, health or points.
