Free To Play (ftp) Continuum is a 2D space shooter since 1995 (aka Sniper, Subspace). And has been quite a nice passtime for over ten years.
The 5MB client (up to about 10MB depending on how many free zones/arenas one adds) is playable even on low end computers with 56K dialup.
FAQ and US/Europe d/l mirrors, beginners guide.
"Meet people from all over the world...then kill them." And it's free!
The longest running massively multiplayer spaceship shooter game running today.