This game is awesome. It's a 2D platformer with RPG elements (like the Wonderboy series) and it's loaded with guns, explosions, jellyfish, tree monsters, jetpacks, and all manner of things that make a great game. Plus, it's got three different endings (the last of which is pure hell trying to obtain). Download it now.
http://maxdamage.justblank.net/cavestory.htm