Plunk! the marble game by OMG! Brews Games:

Inspired by handcrafted puzzles each Plunk! level challenges you to navigate gorgeous marbles through a wooden labyrinth. Tilt your phone or tablet in all directions and use gravity to roll the marbles past holes and into their targets.

The first levels will test your hand-eye coordination as you maneuver one marble around between and sometimes over fiendishly placed holes. Afterwards the difficulty escalates as two three four or more simultaneous marbles force you to come up with increasingly complex plans.

Plunk! is ad-supported software that comes with 12 free levels. An optional upgrade is available within the application that provides many more levels and removes all advertisements.