Marble Arena
The game is freeware but not open-source;
downloads available for Windows and Mac OS X ... but (sadly) not for linux (yet???).
Quoting Marble-Arena.com:
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Zap your way to the top in this exciting new game. Control your marble around easy to intricate levels, up and down slippery slopes, mastering physics contraptions such as jumppads, air vents and vortexes.
Fight your way through AI controlled marbles and send them flying into each other and towards their doom. The more points you get the bigger you grow and the harder you are to be pushed around by physics mechanisms or other marbles.
Navigate on several types of surfaces, such as ice, glass, water, sand and tarmac. Prove that you have what it takes by finishing levels before the time limit and get rewarded with upgrade points which you can use to improve your marble or change your skin.
Join your friends online and battle it out on every map available in the game. Not enough excitement fighting by yourselves ? Add as many AI controlled marbles as you need to make the game really intense.
The arena awaits YOU !
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Quadropolis has added this game into the selection list,
so you can now upload media for it :)