valve

http://www.valve-erc.com/
the best forums on editing every aspect of the Half-Life engine, updated every half-hour

http://croteam.com/
the best forums on editing every aspect of the Serious Engine, updated every half-hour

http://nevrax.org/
home of the disproportionately excellent 3D engine, NEL

flipcode

http://www.flipcode.com
indispensible resources, community hub and articles. also see the flipcode 3D engine list

http://www.cfxweb.net
indispensible resources, community hub and articles. intersects with the demo scene

http://www.gamedev.net/
good resources, articles and book-lists

gmax

http://www.discreet.com/products/gmax/
discreet's customisable level editor package. designed to be flexible enough to meet the needs of a variety of projects

fmo

http://www.fmod.org
possibly the most flexible and best performing audio module for game engines. free for non-commercial projects

http://sourceforge.net/softwaremap/trove_list.php?form_cat=80
the spring from which all Opensource games flow

openal

http://www.openal.org/home/
home of the Open Audio Library

linux

http://linuxgames.com
there you have it

blender

http://blender3d.org
open source: blender is a particularly interesting environment for anyone moving into modelling but also into game design, as it is simultaneously a modeller level editor *and* an engine