User David G thought it'd be a good idea to start a thread with links to various resources that could be used in games.
(Taken from a forum thread - https://app.roll20.net/forum/post/289/game-resources#post-33025 )

Game Systems

FANTASY GENRE -
Dungeons & Dragons - http://www.wizards.com/dnd
Pathfinder - http://paizo.com/pathfinderRPG
Pathfinder Reference Document - http://paizo.com/pathfinderRPG/prd/
Shadowrun - http://shadowrun4.com

SUPERHERO / SCI-FI GENRA -
Danger Patrol - http://dangerpatrol.com
Mutants and Masterminds -
Champions -

HORROR GENRA -
Call of Cthulhu - http://www.chaosium.com/
World of Darkness - http://www.white-wolf.com/

GENERIC or MULTI GENRA
GURPS - http://www.sjgames.com/gurps/
Fiasco - http://www.bullypulpitgames.com/games/fiasco/

Libraries

http://www.opengameart.org - Graphics, Sound Effects, etc
http://www.pdsounds.org - public domain sounds - sound library
http://rpgmapshare.com - maps
http://www.d20srd.org/ - D20 3.5E System Reference Document

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 (can export as jpg, png, or pdf)
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

http://iplay4e.appspot.com/characters - D&D 4E character sheet service
http://www.asmor.com/scripts/4eMagicItems/randomTreasure.php - Treasure Generator
http://www.asmor.com/scripts/4eMagicItems/index.php - Treasure Index
http://4eturntracker.com/ - Combat Manager
http://1d4chan.org/wiki/Drama_Cards - "Drama" / "Bonus" Cards
http://www.power2ool.com - D&D 4E Info Tracker

Books

http://www.amazon.com/X-Treme-Dungeon-Mastery-Tracy-Hickman/dp/0977907465