Doom 3
Doom 3 is a science fiction horror video game developed by id Software and published by Activision. It is a reboot of the DOOM franchise.
System Requirements
Patches
Official Patches
Windows Patch 1.3.1
Linux Patch 1.3.1
OSX Patch 1.3.1
Expansion
Mod Support
Doom 3 has a small, but very knowledgeable mod community that is still very active and covers most id tech based games.
Tools and Information
Doom 3 World - An unofficial id tech community
Modwiki - An unofficial id tech informational wiki run by members of Doom 3 World
iddevnet - Official, but limited, Doom 3/Quake 4 informational site
Doom 3 Mod Lists
Moddb - Best for still active mods
Planetdoom - Some Doom 3 news and archives of non-active Doom 3 mods
D3files - Mostly for archives of non-active Doom 3 mods
Open Source Version
The source code to the engine was released under the GPL license in November 2011. There have since been several modifications. You can view the different branches and changes, as well as download them, from the github repository, if you are willing to compile the code yourself. The same group that worked on ioQuake3 has said they will add features to the code and release compiled binaries under the title ioDoom3. You will have to obtain the art assets separately.