Absolute Blue is a fine horizontal shooter. Shoot stuff, collect upgrades and beat some big bosses. Nice!
From the Homepage:
You have to fight your way through 60 sectors of 4 graphically different worlds with 3 levels each. You can improve your ship with many upgrades and power-ups to transform it into the devastating battle force necessary to confront the endless waves of heavily armored enemy fighters and creatures that will cross your path…
Info:
The game was Shareware until 2009 when it was released as Freeware.
Link to Gameplay Video (Pixel Prospector)
looks awesome!