Grid Wars

Grid Wars is a freely distributed clone of Geometry Wars made by Mark Incitti. It was previously available from his website, but was taken down from there after its popularity drew ire from Bizarre Creation's Legal team.
 PC

Differentiating characteristics

Screenshot of the Geometry wars-like aesthetic
Screenshot of the Geometry wars-like aesthetic
Grid Wars does make some subtle mechanical changes to the Geometry Wars formula, that changes the gameplay from it's obvious source of inspiration.

The biggest change is that black holes play a much bigger role in GridWars. A black hole is a shape on the gridded plane in which the game takes place that exerts gravity on its surroundings and sucks other shapes into it's center, growing in size until it blows up into a cloud of quick, deadly enemies. The twist is that when you take down a black hole (which takes some effort), you get points based on how many shapes the hole has consumed, and the points you get far overshadows the points you would have got from simply shooting the consumed shapes in the first place. Therefore "black-hole farming" (i.e. lead a bunch of shapes into the hole, shoot it until it shrinks to a more manageable size, rince and repeat for as long as you dare, then finally blowing it up) is the only feasible way to play the game in the long run, which makes for a very different experience then what Geometry Wars offers.