xkobo is a great open-source game where you control a tiny space ship in a 2d scroller space.
You are trying to destroy these giant fortresses that are made of nodes which branch out like plants- when you destroy a node, the bars which form the fortress dissolve back to the branch from which that node sprung. Only the nodes themselves can shoot at your ship.The original xkobo uses the X11 libraries and can therefore be played on any Linux or Apple (Mac OS X) system. Highly addictive! Web link of note: xKobo