Space Alien Invaders
Free arcade style space shooter game for the PC.
http://www.freewareshareware.net/download/sai.exe - 1Mb - Download - Screenshot

Invasion 3D
Invasion 3D is a free multi-platform 3D arcade game, based on OpenGL and SDL and available for Windows and Linux. This fun and addictive game features realistic physics simulation, challenging gameplay, stunning visual effects,multiple weapons, hidden bonuses, and more. ... Invasion 3D was Inspired by Taito's original Space Invaders arcade game from 1978, and is dedicated to the MAME arcade emulator system. ... Create massive destruction and rack up as many points as you can as you blast your way through 20 waves with thousands of aggressive space invaders!
http://www.fascinationsoftware.com/FS/media/Invasion3D.tar.gz - 2Mb - Download - Screenshot

Alien Invaders Attack!
Free arcade style space shooter game for the PC.
http://www.sci-fi-game.com/download/aia.exe - 4Mb - Download - Screenshot

Typing Invaders - Free Typing Game
Typing Invaders is a REAL arcade game, in which you are escaping from invaders, shooting and trying to hit invaders. This is no fake - you are playing space invaders ! (You are not just typing characters). With this free typing game you WILL have fun, but don't worry, by playing Typing Invaders... Improve your typing skills with this free typing tutor - typing game !
http://www.giletech.net/Typing-Tutor/TInv_Setup.exe - 1Mb - Download - Screenshot

Star Reaver
Star Reaver is a freeware arcade style space shooter game, available for free download. Defend your ship from dive bombing alien space invaders. Grab the power ups for powerful new multi-directional weapons that shred the marauding invaders. Free download available for PC and Mac.
http://www.freewareshareware.net/download/sr.exe - 2Mb - Download - Screenshot

AstroRaid
Add one part Space Invaders and one part Galaga, then shake with twist of Asteroids to get AstroRaid, a modern version of an arcade space shooter. The fate of humanity is in your hands! The Earth is in the danger of being destroyed by a fast spreading alien virus that kills anybody it infects. ... First, someone tried to kill you while you were inside the space station. ... The only hope is a vaccine that was developed on a remote space station by our scientists. ... Just keep your thumb close to the Fire button.
http://www.gamerange.com/files/astroraidinstall.exe - 2Mb - Download - Screenshot

phoenicas
After a successful alien attack on earth, only 6 humans escaped. Five are onboard non-firing space vessels and one (you) is piloting the only remaining fighting craft. Your mission is to guard the vessels on the journey to safety. Aliens will attack the convoy and try to kill you and abduct the... Use of a proper arcade joystick like hotrod or x-arcade is highly recommended. ... Phoenicas is an arcade musical 2d shooter in the styles of defender, exerion, phoenix, ikaruga and space invaders. ... The defender-like mechanic of protecting humans and rescuing them right from the hands of the...
http://www.phoenicas.com/phoenicas/phoenicas_demo.exe - 16Mb - Download - Screenshot

Star Defender II
Star Defender II is a Space Shooter Arcade. It is developed like an advanced clone of old classic Galaga and possesses all the best features of this genre. Again in this fabulous Shoot'em up game you will get a chance to destroy enormous armies of the cruel invaders and to save the Galaxy! ... Every new mission has different background witch puts you into the atmosphere of the real Space Battle. ... Your small but manoeuvrable space ship is controlled by your mouse. ... And you can enjoy all of this for hours being along or together with your friends and family.
http://www.awem.com/download/StarDefender2Setup.exe - 8Mb - Download - Screenshot

Clash N Slash
Enjoy non-stop action space shooter with extremely simple controls suitable for all ages! Clash'N Slash, a totally new and highly addictive arcade space shooter for players who want to enjoy a fresh exciting gameplay and bored with playing tons of indistinguishable shooter games. ... In Clash'N Slash arcade game you must defend your planet from pesky alien invaders that will arrive from all around the galaxy. ... Loads of weapons, upgrades, enemies will hold you for a long time and innovative bonus triggering system will make your game experience colorful and fun.
http://www.enkord.com/download/clash_n_slash_111.exe - 5Mb - Download - Screenshot

Robot Invasion
Blow away ruthless robot invaders by the hundreds in this fast paced 3rd person 3D shooter. You'll be battling robots in diverse and awesome environments, while trying to save as many humans as possible. Some of the places you'll do battle are a spooky castle, ghastly grave yard, a space station... Pure adredaline filled action awaits anyone who takes on the mission to save humanity in this fast paced action arcade game. ... Just as you get overwhelmed with robots coming at you from every direction, you may get lucky and find a time freeze module, or an EMP blaster to aid you in your fight.
http://www.best3dgames.net/roboinv/roboinvd.zip - 13Mb - Download - Screenshot