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

• Covert Ops -- by DWD Studios
• 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
• Castles & Crusades
• Dungeons & Dragons
• Dungeons & Dragons 3.5E System Reference Document
• Dungeon World
• Pathfinder
• Pathfinder Reference Document

SUPERHERO / SCI-FI GENRE

• Champions
• Danger Patrol
• Mutants and Masterminds
• Star Frontiers (free remastered edition)

HORROR GENRE

• Call of Cthulhu
• World of Darkness

GENERIC or MULTI GENRE

• Fiasco
• GURPS
• Shadowrun

Libraries

• http://www.opengameart.org - Graphics, Sound Effects, etc.
• http://rpgmapshare.com - maps
• http://www.game-icons.net/ - online repository providing heaps of cool game related graphics(Creative Commons)

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
• http://www.gozzys.com - Random Map Generators for Dungeons, Caves, and Wilderness.
• http://donjon.bin.sh - Random Generators including Maps of various varieties.

Sounds

• http://audacity.sourceforge.net - open source sound recorder/editor
• http://www.freesound.org - sounds
• http://www.pdsounds.org - public domain sounds - sound library
• http://sounddogs.com/ - sound effects
• http://tabletopaudio.com/ - Original, 10 minute ambiences and music for your tabletop role-playing games.

System Specific Tools

D&D 4E

• D&D 4E character sheet service
• Treasure Generator
• Treasure Index
• Combat Manager
• "Drama" / "Bonus" Cards
• D&D 4E Info Tracker
• http://donjon.bin.sh/4e/ - random generators

D&D 5E

• http://donjon.bin.sh/5e/ - random generators

Pathfinder

• http://donjon.bin.sh/pathfinder/ - random generators
• http://pathfinder-treasure-generator.com/ - treasure generator

Traveller 5th-edition

• http://heldenhaufen.de/T5 - SectorMaker, SystemMaker, BeastMaker, GunMaker
• http://eaglestone.pocketempires.com/ - several utilities including Task Roller and ship conversions to T5.

Books

• XDM X-Treme Dungeon Mastery, by Tracy Hickman

See Also

• Game Resources in the Off-Topic forum archive