half life 1 used quake 2 engine.
id released quake 2 engine under gnu/gpl licence back in 98/99, valve added new ai and lighting and called it source. all editing still done on the same principle as quake 2. models are still pretty basic compared to doom3 and unreal warfare engines.

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80 years is a long time to acting a fool
quake 4 militia mod http://c0re.vndv.com/