I'm a sucker for any game that features 3D graphics (it goes back to the days of Stunt Car Racer on the Amiga) and especially those that are free.
Marble Arena ticks both boxes and while it's not exactly the most spectacular or exciting game around (certainly not a patch on Stunt Car Racer) Marble Arena is a lot of fun. Basically, Marble Arena requires you to roll your marble around a 3D obstacle course going up and down slippery slopes, mastering air vents, jump-zones and even vortexes.
The more obstacles you successfully navigate in Marble Arena, the further you get and the more points you score. You can face a variety of surfaces such as sand, water, glass, ice and tarmac and if you get through them all, you're rewarded with upgrade points which you can use to improve your marble or change your skin.
It can be a bit boring on your own after a while although you do quite attached to the ball's plight. However, I imagine the real fun starts when you add more balls and more players who can roll against each other at the same time.
Marble Arena will either make you roll over and go to sleep after a few levels or lose your marbles trying to navigate it's many challenges.