Volfied (similar but much better to Qix)
Discs of Tron
Mr Driller
Omega Race
In the Hunt (pretty cool submarine side scroll shooter)
Bombjack (more for the music than the gameplay)
Kangaroo (kinda like popeye and donkey kong mix)
Bubble Bobble
edit forgot:
Rolling Thunder
Slap Fight (this is a great shooter with power ups and awesome easter eggs. Get to a certain point of the first level without shooting and after you die you respawn with a fully powered up ship.)