Free

AQ2
Based on the Quake 2 engine. Offers screenshots, downloads, forums and progress reports.
http://www.cam-direct.co.uk/ltk/ltktbm.shtml

Baroq
Arcade shooter for Windows with multiple power-ups that combine for more effective weapons.
http://www.sting.co.jp/play/baroshu/index2_e.htm

Crucible of Stars
Science-fiction first person shooter game. Includes downloads and weapon specifications.
http://crucibleofstars.sourceforge.net/

Dave Gnukem
Side scrolling shooter for Windows or Linux. (Open Source, GPL)
http://gnukem.sourceforge.net/

D-Day Normandy
WWII shooter based on the Quake 2 engine. Offers downloads, screenshots and editing tools to download.
http://dynamic.gamespy.com/~ddc/site/

Dstar
Multiplayer game in which players shoot each other with lasers and missiles. Includes downloads, instructions and screenshots.
http://dstar.sourceforge.net/

FireCrow
A shoot 'em up game for Windows. Includes information and downloads.
http://firecrow.ifrance.com/firecrow/

Gaming's Evolution Team
Download free games and minigames.
http://www.freewebs.com/ge-team/

HalfLight
Scrolling spaceship shooter for Windows. Game description and backstory, with links to reviews.
http://gapingwolf.com/halflight/

Just Shoot the Thing
Shockwave shoot-em up for Windows featuring shields, bonus ships, powerups, 50 Sound effects, 10 built in targets, 3 different weapons and the ability to take any image from the clipboard and use it as the target.
http://www.digitalliquid.com/dldata/soft.html#just

The Melody of Heaven Shooter Games
Includes two arcade-style clones: Cosmocrafter and Sea Sug Invasion.
http://www.freewebs.com/melodyofheaven/games.htm

Meteor
Military shooter for DOS in which you control the main character in real-time.
http://www.jbgames.com/games/meteor/

Mortar Mayhem
A shooter for up to eight players for DOS or Windows.
http://jj.starthotel.dk/mortar/

My Blitz Basic Programs
Source code available for Ms Blitzman, Scrabble, and Blitzum. Also has downloads for Deblitzer, Blitzatron, and MMMarbles.
http://www.incitti.com/Blitz/

Ogrian Carpet
Use a flying carpet destroy enemies and build a fortress on each level. Includes downloads, screenshots and forums.
http://ogrian.sourceforge.net/

Open Quartz
Describes 3D shooter based on the Quake engine, with source, maps, textures, and utlilities. (Open Source, GPL)
http://openquartz.sourceforge.net/

Robotech Lab
Mech Combat simulation. Offers downloads, forums and screenshots.
http://gerwalk.net

Scope Strike
Sniping game. Includes a download, overview and screenshot.
http://www.scopestrike.com/

Shadow
Offers download and version history.
http://www.freewebs.com/shadow-game/

Space Plumber
First-person 3d game for Linux, not much different to Doom and Quake.
http://www.triptico.com/splumber/

Wulfram II
Online 3D shooter and strategy game allowing up to 64 players per server.
http://www.wulfram.com/

X-Bomber
2D 32-bit scrolling shooter for Windows, based on Japanese Science Fiction TV series "Star Fleet." News, screen shots, game background, and discussion forums.
http://www.xbomber.co.uk/