The engine that powered the sleeper hit TRIBES and TRIBES2. It is a very solid professional grade engine with
excellent community support. Find more here
Quake / Quake II Game Engines
ID Software has donated the source code for Quake I and Quake II, their evergreen, hallmark engines to the general community under the GPL.
If you want to get under the hood in one of the most successful engines ever.
open source, free (as in beer) and with a very active community. Check it out
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