I can't really complain too much as this IS a free game, but here's just some things that could be improved:
-Graphics: These look rather basic, and the main menu's black background is a bit bland. Font is quite simplistic. Perhaps consider a logo for the main menu?
-Audio: The sound effects are actually quite satisfying, but consider putting in a soundtrack etc.
-Gameplay: Of course it's basic, but the physics are very good. Occasionally the balls get stuck, however. Consider adding new modes, for example Survival (drop a ball, lose a life - x number of lives), Speed (1 minute, score as high as you can) and endless (essentially what is already included).