ETC2 is a single player mod made by DaveJ for Half Life. Set within the Half Life universe, it brings back Gordon Freeman to fight against aliens, zombies and soldiers.
This is a rare working copy of ETC1, hard to find these days!
etc1 (fixed) full version
DaveJ released the source files for ETC2, so that mappers could modify the game. What are you guys waiting for? Port the maps to the Source engine! :D
etc2 source files for mappers. source code