Tremulous is a free and open source asymmetric team-based first-person shooter with real-time strategy elements. The game features two opposing teams: humans and aliens. Each team must attack the enemy’s base and team members while defending their own base.
https://tremulous.net/
I'd actually recommend Unvanquished instead! It's a fork of Tremulous that has more active development, including a rehaul of the rendering engine. https://unvanquished.net
Reminds me of Natural Selection 1/2. Loved those games and I wonder if this was their inspiration. And of course they ended up making Subnautica with their expertise in creature design.