Whilst I was looking at those other games I came across a game which I bookmarked before called AstroMenace from an independant games company in the Ukraine called ViewWizard Games but just found out there is a free version for Linux. I played this for a good couple of hours and was really hooked. The screenshot doesn't do it justice.
AstroMenace at first seems like your run of the mill arcade shooter. Well it is but a difference. For one the graphics are awesome. It is 3D with a slight perspective. Using your mouse you push forward and your ship shrinks into the distance. The enemy ships move around in complete 3D environment, although you stay on the same plane. Instead of powerups collected in most games of this type you earn money during missions. This money can be used to upgrade your ship with various weapons and system upgrades, like better tracking computer.
I have done about 10 of this missions but went back to start from the first mission to earn more money to upgrade as the later levels as you'd expect get harder.
This is a shareware game and you need to pay for the Windows version, only about 10 quid. I think I will donate some money as it is a pretty cool game and well worth it.