Hyperoid

Hyperoid is a freeware port of the original Windows 3.1 Hyperoid game by Edward Hutchins.

This is still a single player Asteroids clone but while this is a port of an earlier game the gameplay has been modified, an extra ship is awarded when the player scores 33,333 points or a multiple thereof, and the baddies' bullets do more damage.

The game plays in a resizeable window and, allegedly, it can even be played in the game's icon. It is entirely keyboard controlled and has a 'Boss key'.