Alien Pool is a compelling space shooter written in 
C++, and using SDL. It supports many interesting features:

Colorful ray-traced graphics.
Immersive sound effects.
Digitally sampled music.
Joystick control.


Alien Pool is similar to both asteroids and pool.
Move a spaceship around the screen and shoot at aliens.
Aliens that have been shot bounce around the screen 
and hit other aliens. Collect power-ups that provide 
extra health, hyperspace, and supershoot!

Alien Pool is free software released under 
the GNU GPL.

Screenshots + source:
http://mike.taequin.org/alienpool/