If you grew up playing quarter munching arcade games in the 80’s chances are good you have a taste for a well designed shmup, and if you do, you owe it to yourself to check out Shoot the Core and most especially their database of available shmups, currently listing 950 games and growing all the time. My fave for today is APOCALYSPE: DEUS EX MACHINA, a robotron-inspired 8 way shooter.
Almost all of this stuff is for the PC, sorry mac/linux folk. A lot of it is also demo or commercial, but there are free and open source ones in there and some of the demos are very generous. Besides, some of them are well worth the purchase price.