Astro Lander - Fly a space rocket through the asteroid fields.
Moon Rocks - Build a moon base and try not to be destroyed by the errant meteorites.
Space Invaders - A classic game of survival in a world of hostile aliens.
Astro Joe - Take the role of the alien in this quirky little rocket game.
SeaSky - A Site with a good selection of learning resources and games with Space and Sky related themes. Including action games, word searches and puzzles.
Nasa's Student Resource Pages - A good site to look at for ideas for your moonbase.