Maelstrom is an old-school Asteroids clone with plenty of extra features. While the program was originally published by Ambrosia Software for Mac OS Classic and is still available for sale on their website, the game's author has released Maelstrom 3.0 separately as open source.
Gameplay involves endless levels of shooting asteroids, where the player cannot progress to the next level until everything that can be destroyed has been destroyed. Being hit by any obstacle results in instant death, but the player can activate an expendable shield to protect themselves from harm. It's also possible to gain extra lives with every certain number of points.
Non-asteroid objects are frequent and include steel balls that can't be broken apart, flying saucers that shoot at the player, and bonus multipliers. The faster the player completes a level, the higher the bonus.
Sam's Protip: Just to clarify, the game's icon is a picture of an asteroid. I thought for the longest time that it was an image of a sagittal brain slice.