This is one of the most fun games I've played in a long time. It is a remake of invaders but don't let that throw you off, it isn't a simple or easy game. There is really cool ship upgrade system, boss ships, tons of different enemies, really cool techno style music (if you've got a sub you'll love it), an online high score table... and the best part is its only $10. The demo gives you about an hour of play time... check it out:
http://www.puppygames.net/titan-attacks/index.php
http://www.puppygames.net/titan-attacks/index.php