I thought it'd be a good idea to start a thread with links to various resources that could be used in games. These could be graphics/sound programs, libraries, informational, etc.
Post any you find useful and I will add them to the list.
Libraries
<a href="http://www.opengameart.org" rel="nofollow">http://www.opengameart.org</a> - Graphics, Sound Effects, etc
<a href="http://www.pdsounds.org" rel="nofollow">http://www.pdsounds.org</a> - public domain sounds - sound library
<a href="http://rpgmapshare.com" rel="nofollow">http://rpgmapshare.com</a> - maps
<a href="http://www.d20srd.org/" rel="nofollow">http://www.d20srd.org/</a> - D20 3.5E System Reference Document
<a href="http://paizo.com/pathfinderRPG/prd/" rel="nofollow">http://paizo.com/pathfinderRPG/prd/</a> - Pathfinder Reference Document
Graphics & Mapping
<a href="http://www.blender.org" rel="nofollow">http://www.blender.org</a> - open source 3d graphics software
<a href="http://www.gimp.org" rel="nofollow">http://www.gimp.org</a> - open source 2d graphics software
<a href="http://pyromancers.com" rel="nofollow">http://pyromancers.com</a> - free online map maker (can export as jpg, png, or pdf)
<a href="http://rpgvirtualtabletop.wikidot.com/tokens" rel="nofollow">http://rpgvirtualtabletop.wikidot.com/tokens</a> - tokens (free & paid)
<a href="http://www.ugo.com/games/superhero-generator-heromachine-2-5" rel="nofollow">http://www.ugo.com/games/superhero-generator-heromachine-2-5</a> - superhero creator
<a href="http://www.rptools.net/index.php?page=tokentool" rel="nofollow">http://www.rptools.net/index.php?page=tokentool</a> - Token Creator
<a href="http://sourceforge.net/projects/pymapper/" rel="nofollow">http://sourceforge.net/projects/pymapper/</a> - Map Maker
Sounds
<a href="http://audacity.sourceforge.net" rel="nofollow">http://audacity.sourceforge.net</a> - open source sound recorder/editor
<a href="http://www.freesound.org" rel="nofollow">http://www.freesound.org</a> - sounds
<a href="http://sounddogs.com/" rel="nofollow">http://sounddogs.com/</a> - sound effects
System Specific Tools
<a href="http://iplay4e.appspot.com/characters" rel="nofollow">http://iplay4e.appspot.com/characters</a> - D&D 4E character sheet service
<a href="http://www.asmor.com/scripts/4eMagicItems/randomTreasure.php" rel="nofollow">http://www.asmor.com/scripts/4eMagicItems/randomTreasure.php</a> - Treasure Generator
<a href="http://www.asmor.com/scripts/4eMagicItems/index.php" rel="nofollow">http://www.asmor.com/scripts/4eMagicItems/index.php</a> - Treasure Index
<a href="http://4eturntracker.com/" rel="nofollow">http://4eturntracker.com/</a> - Combat Manager
<a href="http://1d4chan.org/wiki/Drama_Cards" rel="nofollow">http://1d4chan.org/wiki/Drama_Cards</a> - "Drama" / "Bonus" Cards
<a href="http://www.power2ool.com" rel="nofollow">http://www.power2ool.com</a> - D&D 4E Info Tracker
Games
<a href="http://www.wizards.com/dnd" rel="nofollow">http://www.wizards.com/dnd</a> - Yeah, D&D
<a href="http://paizo.com/pathfinderRPG" rel="nofollow">http://paizo.com/pathfinderRPG</a> - Pathfinder
<a href="http://shadowrun4.com" rel="nofollow">http://shadowrun4.com</a> - Shadowrun
<a href="http://www.chaosium.com/" rel="nofollow">http://www.chaosium.com/</a> - Call of Cthulhu
<a href="http://www.white-wolf.com/" rel="nofollow">http://www.white-wolf.com/</a> - World of Darkness, etc
<a href="http://www.bullypulpitgames.com/games/fiasco/" rel="nofollow">http://www.bullypulpitgames.com/games/fiasco/</a> - Fiasco
<a href="http://dangerpatrol.com" rel="nofollow">http://dangerpatrol.com</a> - Danger Patrol
<a href="http://www.sjgames.com/gurps/" rel="nofollow">http://www.sjgames.com/gurps/</a> - GURPS
Books
<a href="http://www.amazon.com/X-Treme-Dungeon-Mastery-Tracy-Hickman/dp/0977907465" rel="nofollow">http://www.amazon.com/X-Treme-Dungeon-Mastery-Tracy-Hickman/dp/0977907465</a> - XDM X-Treme Dungeon Mastery - awesome book with good tips and even better sense of humor