40ksource.com

moddb.com/mods/exterminatus/

Check these websites at least once every weekend (Sundays from noon to evening GMT normally) for weekly updates to the mod.

The creator is "ChromeAngel". Other contributors include:

DontAsk4470 (textures/skins/effects/sounds)
straydoom (mapper)
Stormwolf 597 (models/voices)
Onslaught Delete (mapper/models)
Berserker (mapper)

Official Steam group: http://steamproxy.net/groups/exterminatusgame