The games suck, but they're free.

[Minzoku]

*is bored*

I'm posting in OT because the few shmuppish ones are probably borderline... anyway, Miniclip.com is a fine way to waste several hours, just perusing the selection alone.

These are the, if anything, single-screen/non-scrolling shmups I found:

Space Fighter [basically Asteroids]
http://www.miniclip.com/spacefighter.htm

Lunar Command [Defender]
http://www.miniclip.com/lunarcommand.htm

Space Invaders [updated Space Invaders]
http://www.miniclip.com/spaceinvaders.htm

Galactic Warrior [a crappy Galaga]
http://www.miniclip.com/warrior.htm

Starship Eleven [Lunar Lander]
http://www.miniclip.com/starshipeleven.htm

How I found the site in the first place:

Miniclip Tetris
http://www.miniclip.com/tetris.htm


Edit: Oh, wait, try the shoot-em-up page instead... Platypus is on there, though some of the others I question.