For those picky concerning graphical sophistication and professionalism, there's always the free multiplayer component of F.E.A.R.:
http://www.joinfear.com/main

I fondly remember playing this shmup, Tyrian back in the waning days of DOS Shell; it was awesome shareware.
Now it's free, but damned if I can get it working right.
http://members.iinet.net.au/~vannevar/tyrian/downloads.html
You'll likely need dosbox to run it, if you don't already have it.
http://dosbox.sourceforge.net/download.php?main=1