Do you like ancient 2D games? If so, you have to play cave story, or Doukutsu Monogatari as it's called in Japanese. It's a Japanese (you can get the translated English version tho) side-scrolling adventure developed by Studio Pixel.
The developer Pixel spent five years making this game as fun and as great as it is by adding solid controls, smooth gameplay, a great cast of characters and dialog, an interesting plot, good music and beautiful oldskool 2D artwork.You play the part of a small robot stranded on an island in the sky with a mission to defeat a mad doctor and his subordinates. There are multiple endings, several boss battles, tons of items to collect and all kinds of added secret bonuses and special challenges. It's a great great game, probably one of the best I've ever played.