Here are a few resources you can use for a variety of games. This list is not comprehensive.

Gaming Systems

MODERN ACTION/ADVENTURE/SPY/MILITARY GENRE

• d20 Modern - by Wizards of the Coast
• Feng Shui- based on Hong Kong/martial arts movies
• GUMSHOE System - by Pelgrane Press
• James Bond 007 - by Victory Games [out of print] - based on the Ian Fleming character
• Mercenaries, Spies and Private Eyes - Designed by Michael Stackpole.
• Over the Edge - by Atlas Games
• Spycraft - by Crafty Games - espionage/adventure
• Modus Operandi - Espionage RPG webring - for all things spy...

FANTASY GENRE

• Basic Fantasy RPG
• Dungeons & Dragons
• Dungeons & Dragons 3.5E System Reference Document
• Pathfinder
• Pathfinder Reference Document
• Castles & Crusades

SUPERHERO / SCI-FI GENRE

• Danger Patrol
• Mutants and Masterminds
• Champions

HORROR GENRE

• Call of Cthulhu
• World of Darkness

GENERIC or MULTI GENRE

• GURPS
• Fiasco
• Shadowrun

Libraries

• http://www.opengameart.org - Graphics, Sound Effects, etc
• http://www.pdsounds.org - public domain sounds - sound library
• http://rpgmapshare.com - maps

Graphics & Mapping

• http://www.blender.org - open source 3d graphics software
• http://www.gimp.org - open source 2d graphics software
• http://pyromancers.com - free online map maker
• http://rpgvirtualtabletop.wikidot.com/tokens - tokens (free & paid)
• http://www.ugo.com/games/superhero-generator-heromachine-2-5 - superhero creator
• http://www.rptools.net/index.php?page=tokentool - Token Creator
• http://sourceforge.net/projects/pymapper/ - Map Maker

Sounds

• http://audacity.sourceforge.net - open source sound recorder/editor
• http://www.freesound.org - sounds
• http://sounddogs.com/ - sound effects

System Specific Tools

• D&D 4E character sheet service
• Treasure Generator
• Treasure Index
• Combat Manager
• "Drama" / "Bonus" Cards
• D&D 4E Info Tracker

Books

• Dungeon Mastery, by Tracy Hickman

See Also

• Game Resources in the Off-Topic forum archive