If you already have your game running, probably the next thing you want to do is add some sound effect and music. If you are just programmer who can not compose music, you usually want to search the internet for those.
Learn how to make your game entity explore a large game area but remains in the center of the screen.
A small application to generate city names. Pretty nice. It can generate any number of city names depends on your need.
Parallax scrolling is a simple yet beautiful effect to give 3D feel on your 2D projects. This effect is usually applied in platformer or shoot em up games. Adventures Of Tinger is an example of the game that uses this effect. Learn how can you make one here.